Drug interactions involving warfarin

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PRACTICE TOOL
PEER-REVIEWED
Drug interactions involving
warfarin: Practice tool and
practical management tips
Tammy J. Bungard, BSP, PharmD; Erin Yakiwchuk, BSP, ACPR; Michelle Foisy, BScPharm,
PharmD, FCSHP; Cynthia Brocklebank, PharmD, ACPR
Introduction
Warfarin has been the mainstay of oral anticoagulant therapy for the past 60 years and it is most
commonly used to treat or prevent thrombosis or
thromboembolism in patients with venous thromboembolism, atrial fibrillation and prosthetic heart
valves.1 However, this drug is efficacious only when
the dosage is maintained within a narrow therapeutic index, measured by the international normalized ratio (INR). Multiple challenges exist in
appropriately achieving and maintaining therapy
within this narrow index. Recent data have identified genetic variants that may reduce a person’s
requirement for warfarin.2,3 Furthermore, once a
suitable dosage of warfarin has been established,
control of therapy can be affected by changes in
intake of vitamin K, development of acute medical
conditions (e.g., fever, diarrhea), changes in certain
chronic medical conditions (e.g., heart failure) and
interactions with prescription, nonprescription
and herbal products.1
The numerous drug interactions involving warfarin are well known among health care providers.
Although community pharmacies have software
systems that will alert users to a potential interaction, these systems are limited in their capacity
to assist the clinician in managing the interaction.
Similarly, many published reports provide lists of
drugs that are likely to interact with warfarin, along
with their directional impact on the INR, but only
rarely do they offer suggestions for managing the
interaction.1,4 Data on the timing of the onset (and
offset, namely the removal of the interacting drug
with washout phase) of an interaction with warfarin, in addition to the mechanism, are needed to
assist the front-line clinician in effectively managing common drug interactions. As such, we sought
to compile a practical, user-friendly practice tool
that clinicians could use to proactively manage
the care of individual patients during concomitant therapy with warfarin and drugs known to
interact with warfarin.
Development of the practice tool
The practice tool for drug interactions involving
warfarin presented here was originally developed
by a single practitioner (T.J.B.) working in an
anticoagulation management service, who began
charting common interactions between warfarin
and other prescription drugs and the management strategies used to address them. A pharmacy
resident (E.Y.) further developed this chart by conducting a formal search of the literature to compile
a list of established interactions between warfarin
and other drugs. Despite this literature review, it
must be emphasized that the information in the
practice tool reflects years of experience in the
tracking and proactive day-to-day management of
common interactions by the 2 largest anticoagulation clinics in Alberta, namely the Anticoagulation
Management Service at the University of Alberta
Hospital and the Calgary Zone Anticoagulation
Management Service. Interactions specific to HIVrelated medications were reviewed and refined by
a pharmacist practising within this area (M.F.).
The practice tool does not include all documented
interactions with warfarin, but instead focuses
on those for which clinical management strategies have been employed by the 2 anticoagulation
management services. For interactions that are not
proactively managed within these services yet are
likely to be observed, evidence-based (as opposed
to experience-based) information is presented.
To ensure practical applicability, the practice
C P J / R P C • JA N UA RY / F E B RUA RY 2 0 1 1 • VO L 1 4 4 , N O 1 21
tool contains information on aspects deemed
pertinent in assessing and managing interactions
for individual patients, including the effect on INR
(in terms of both directional trend and severity),
mechanism of the interaction, time of onset, offset
and suggested management strategies employed
by the anticoagulation management services. We
included only drugs with established interactions,
(i.e., we excluded theoretical interactions) and we
did not include interactions with chemotherapeutic agents. Furthermore, we excluded herbal
preparations and supplements, because of their
heterogeneous nature. It should be acknowledged
that some disease states, such as those producing alterations in thyroid function, will alter the
metabolism of clotting factors, which can, in turn,
affect warfarin requirements. A detailed review
of these drug–disease interactions is beyond the
scope of this article.
The practice tool was reviewed by the clinical
pharmacist leaders and interested staff of 2 anticoagulation management services in Alberta. Because
the 2 clinical pharmacist leaders spearheaded the
development of intellectual content for provincial
educational programs focusing on anticoagulation
management, a portion of the tool was integrated
into those programs. Specifically, a portion of an
earlier version of the tool was included in a paperbased overview entitled “Anticoagulation: On the
Road to Practice Change” and another version was
included as a handout for individuals attending a
2-day workshop administered through the Office
of Continuing Pharmacy Education, Faculty of
Pharmacy and Pharmaceutical Sciences, University of Alberta, in the fall of 2009.5 Since then, the
practice tool has been updated as described above.
Mechanisms of drug interactions
Warfarin is well absorbed, is highly bound to the
plasma proteins (99%) and is metabolized via the
cytochrome P450 system.1 It is an indirect anticoagulant, exerting its effect by preventing the internal recycling of oxidized vitamin K to reduced
vitamin K. Reduced vitamin K is necessary to
enable carboxylation of the terminal g-glutamic
acid residue of the vitamin K–dependent clotting
factors (factors II, VII, IX and X). The metabolism of warfarin allows for both pharmacokinetic
and pharmacodynamic mechanisms for drug
interactions.
Induction or inhibition of cytochrome P450 isozymes
Warfarin has 2 active isomers.1 The S-isomer is
approximately 2 to 4 times more potent than the
R-isomer and is metabolized primarily by the cyto22 chrome P450 2C9 isozyme (CYP2C9). The R-isomer is metabolized by cytochrome P450 1A2 and
3A4 isozymes. Drugs that induce or inhibit these
enzyme systems have the ability to alter warfarin
metabolism and to decrease or increase the INR,
respectively. Most notable are interacting substances that affect CYP2C9, given the increased
potency of the S-isomer. Interactions involving
these isoenzymes tend to be delayed, for a variety
of reasons. First, the complete effect of the interaction will not be observed until the interacting
agent has reached a steady state (about 5 halflives).6 Second, given the indirect effect of warfarin, time must be allowed for the newly established
“warfarin concentration” to affect the vitamin K–
dependent clotting factors being newly synthesized
by the liver. Concomitantly, clotting factors present
in the circulation before initiation of the interacting agent must be depleted. An effect on the INR is
typically observed within 3 to 5 days for interacting
substances with short half-lives. The effect on the
INR of drugs with longer half-lives will be even
further delayed. Generally, onset and offset occur
over similar intervals. Relative to inhibitory interactions, the onset of induction interactions takes
longer because of the time required for up-regulation (i.e., the process of induction) and synthesis
of new proteins and enzymes. The full impact of
steady-state interactions may not be apparent for
2 to 3 weeks, depending on the inducer involved.
Likewise, when the inducer is discontinued, a
wash-out period of several weeks may be required
before normalization of the hepatic enzymes.
These factors must all be considered when dosing
and monitoring warfarin.6
Displacement of binding with plasma proteins
As noted above, warfarin is highly bound to proteins (primarily albumin) in the plasma and has
the potential for interactions with other highly
protein-bound substances. The effect is usually
transient and the clinical significance may be questionable.7 This type of interaction is probably more
significant in the presence of an enzyme inhibitor, because the body is unable to compensate by
increasing the metabolism of the higher free fraction of the displaced drug.
Alterations in vitamin K status
Dramatic alterations in vitamin K status can affect
the INR. For example, increases in the consumption of vitamin K through the diet or supplements will decrease the INR, whereas reductions
in vitamin K, through decreased consumption
or increased elimination (e.g., diarrhea or medi-
C P J / R P C • JA N UA RY / F E B RUA RY 2 0 1 1 • VO L 1 4 4 , N O 1
cations altering gastrointestinal flora), tend to
increase the INR. Patients should be encouraged to
have a consistent overall consumption of vitamin
K to avoid this type of variation.
Broad-spectrum antibiotics are postulated to
potentiate warfarin by altering the normal intestinal flora, thereby reducing the body’s ability to synthesize vitamin K. However, this factor is unlikely
to be clinically significant for most patients, except
those who are malnourished or have other issues
with malabsorption.7,8
Contribution of hemorrhagic or thrombotic risk
Any medication that impairs the platelets’ ability
to function (e.g., acetylsalicylic acid, clopidogrel,
nonsteroidal anti-inflammatory drugs) and is
given concomitantly with warfarin may increase
the risk of bleeding without affecting the INR.
Conversely, certain medications (such as estrogens) increase the risk of thrombosis and their use
in patients who are taking warfarin must be carefully assessed.9
Clinical use of the practice tool
This practice tool is designed to be used for
patients already taking warfarin who are beginning therapy with a potentially interacting drug.
To apply the practice tool to an individual patient,
the clinician will need some basic information
about the patient. First, assess the individual’s risk
of clotting (according to the indication for warfarin) in relation to his or her risk of bleeding. On
the basis of this assessment, conceptually define
whether you would rather have the patient’s next
INR value above or below the target INR range.
Second, assess the patient’s current anticoagulation
status. Whether the patient is at the higher or lower
end of the therapeutic INR range may influence
your decision about altering the warfarin dosage.
Because many patients take different amounts of
warfarin on different days of the week, changes in
the warfarin regimen are typically determined in
terms of a percentage change in the weekly dose.
Third, review the practice tool to determine the
onset of the interaction and the expected degree
of alteration in the INR (if reported), consider
the need to alter the present dose of warfarin
and estimate the appropriate time for follow-up
INR testing. Finally, take into account the offset
of the interaction, giving consideration to clinical
experience, published reports, the half-life of the
interacting drug (and hence its clearance from the
body) and the mechanism of the interaction (recognizing that induction or inhibition of hepatic
CYP450 isozymes may add to the complexity of
the interaction and result in further delays in the
offset). Once these factors have been accounted
for, it should be possible to estimate the timing of
resumption of the baseline (maintenance) warfarin dose.
We hope that this practice tool will assist clinicians in providing proactive patient care in managing interactions between warfarin and other
drugs. n
From the Anticoagulation Management Service and Division of Cardiology, Department of Medicine,
University of Alberta (Bungard), Edmonton, Alberta; College of Pharmacy and Nutrition, University of
Saskatchewan (Yakiwchuk [student]), Saskatoon, Saskatchewan; Northern Alberta HIV Program, Alberta
Health Services (Foisy), Edmonton, Alberta; Anticoagulation Program, Alberta Health Services (Brocklebank), Calgary, Alberta. Contact [email protected]
Acknowledgements: We acknowledge the following clinical pharmacists, who reviewed and provided clinical
insight into this drug interaction tool: Rene Breault, Tara Carnovale, Kim Fitzgerald, Ellin Jessica Hrudey,
Andrea Pickett and Karen Schultz.
Acknowledgement of competing interests: Dr. Bungard has been on advisory boards for Roche Diagnostics, AstraZeneca, Bayer, Pfizer and Boehringer Ingelheim. She has received travel and speaker honoraria
sponsored by Boehringer Ingelheim and DuPont Pharma. Erin Yakiwchuk has received scholarships from
Merck Frosst Ltd. and Sanofi-Aventis. Dr. Foisy has received speaker honoraria from Abbott Laboratories,
GlaxoSmithKline and Gilead Sciences, Inc. Dr. Brocklebank has received speaker honoraria sponsored by
Leo Pharma and Bayer Inc.
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PRACTICE TOOL Warfarin drug interactions*
Drug
Acarbose10
Acetaminophen10,11
(doses >2 g/d)
Allopurinol10
Direction and
Anticipated Anticipated offset
severity of effect
Mechanism
onset
(t½)†
on INR
2–3 days
NR
↑ INR
Unknown: effect may be
due to increase in warfarin
(t½ = 2 hours)
Moderate
absorption or to drugassociated diarrhea
2–5 days
NR
↑ INR
Decrease in warfarin
metabolism and/or decrease
(t½ = 2–4 hours)
Moderate
in production of clotting
factors
↑ INR
Unknown
3–5 days
Inhibition of warfarin
metabolism;
amiodarone may also
increase or reduce INR by
inducing hyper- or hypothyroidism, respectively
3–7 days
May inhibit warfarin
metabolism (through
CYP3A4 inhibition)
Delayed
Moderate
Amiodarone10,12
↑ INR
Moderate to
severe
Amprenavir‡10,13,14
↑ INR
Moderate
ASA10,15
Atazanavir‡10,14,16,17
No effect at doses Irreversible inhibition of
< 6 g/d, ↑ risk of platelet function
bleeding
Major
↑ INR
Moderate
Azathioprine and
mercaptopurine10,18
↓ INR
1–3 days
May inhibit warfarin
metabolism (through
CYP3A4 inhibition)
Delayed
Possible increase in warfarin
metabolism
1–3 days
Possible decrease in
warfarin metabolism;
interaction is often
compounded by other
factors that may increase
INR (e.g., fever, decreased
appetite)
3–7 days
Moderate
Azithromycin10,19
↑ INR
Moderate
24 Suggested management
Monitor INR closely when starting
or stopping acarbose
Monitor INR when starting
or stopping higher doses of
acetaminophen; minimize use of
drug (e.g., <2 g/d for short courses
[<1 week])
Reports of interaction are
NR
inconsistent; monitor INR when
(t½ = 1–2 hours; for starting or stopping allopurinol
active metabolite,
oxypurinol, t½ = Reassess in 1 week
15–25 hours)
~ 90 days; may
Monitor INR closely (i.e., weekly)
be longer if
when starting or stopping
amiodarone therapy amiodarone; if loading doses of
is prolonged
amiodarone are used, interaction
will occur sooner; AMS considers
(t½ = 26–107 days) empiric 10%–25% warfarin dose
reduction 1 week after starting
amiodarone, in anticipation of
eventual dose reductions of up to
60%
Delayed
Monitor INR more frequently when
starting or stopping amprenavir;
(t½ = 7–10 hours) addition of ritonavir booster
(CYP2C9, CYP1A2 inducer) may
result in net decrease in INR; see
entry for ritonavir for additional
information
5–7 days (inhibitory Use lowest effective dose of ASA; use
effects of ASA on enteric-coated formulation; monitor
platelets last for for bleeding
lifetime of each
platelet)
Delayed
Monitor INR more frequently when
starting or stopping atazanavir;
(t½ = ~7 hours) addition of ritonavir booster
(CYP2C9, CYP1A2 inducer) may
result in net decrease in INR; see
entry for ritonavir for additional
information
NR
Monitor INR when azathioprine
therapy is started or discontinued
(t½ = 5 hours)
or dosage is adjusted; significantly
more (2- to 3-fold) warfarin may be
required when given concurrently
with azathioprine
NR
Inconsistent effect; monitor INR
closely when starting or stopping
(t½ = 68 hours) azithromycin; AMS will not
empirically decrease warfarin unless
patient has other factors affecting
INR (e.g., fever, decreased appetite)
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PRACTICE TOOL Warfarin drug interactions*
Drug
Bismuth subsalicylate10
Bosentan10
Direction and
severity of effect
Mechanism
on INR
Possible displacement of
­↑ risk of
protein binding
bleeding
Moderate
↓ INR
Moderate
Carbamazepine
(CBZ)10,20
↓ INR
Moderate to
severe
Celecoxib10
Cholestyramine10
Cimetidine10
Anticipated Anticipated offset
onset
(t½)†
1–3 days
(t½ = 2–5 hours)
May induce warfarin
metabolism (through
CYP3A4 and/or CYP2C9)
5–10 days
Increase in warfarin
metabolism (through
CYP2C9 induction)
10–35 days
↑ INR
2–5 days
Celecoxib is metabolized
by CYP2C9 but does not
Major (especially inhibit or induce this
in elderly
isozyme
patients)
↓ INR
Decrease in absorption of
warfarin
Moderate
↑ INR
Decrease in warfarin
metabolism
↑ INR
Moderate
Clarithromycin10,23
↑ INR
Moderate
Clopidogrel10
No effect on
INR,
↑ risk of
bleeding
Severe
Cloxacillin10
↑ INR
Moderate
Monitor INR when starting or
stopping bosentan; AMS considers
(t½ = 5–8 hours) empiric 15%–20% warfarin dose
increase, with further increases
according to weekly INR; may need
increase in warfarin dose of as much
as 50%
Delayed (14–40 Monitor INR closely when starting,
days)
stopping, or adjusting CBZ; increase
in warfarin dose of 50%–100% may
(t½ = 12–17 hours) be required when initiating CBZ;
decrease warfarin dose by ~50%
when stopping CBZ
NR
Monitor INR closely when starting
or stopping celecoxib; monitor for
(t½ = 11 h)
bleeding; AMS considers empiric
0%–15% warfarin dose reduction
NR
3–5 days
~1 week
(t½ = 2 hours)
Unknown; may be due
to CYP1A2 inhibition;
interaction more prevalent
among elderly patients
taking multiple medications
2–5 days
Inhibition of warfarin
metabolism (through
CYP3A4 inhibition)
3–7 days
Antiplatelet effects of
clopidogrel combined with
anticoagulant effect of
warfarin impair clotting
Unknown
Avoid this drug, if possible, especially
at high doses; monitor INR and
monitor for bleeding
NR
1–3 days
Moderate
Ciprofloxacin10,21,22
NR
Suggested management
2–4 days
(t½ = 3–6 hours)
NR
(t½ = 5–7 hours)
Monitor INR more frequently when
starting or stopping cholestyramine;
avoid administering cholestyramine
within 2 hours of warfarin
Monitor INR closely when starting
or stopping cimetidine until INR is
stable; consider changing to another
H2RA or PPI instead of using
cimetidine
Monitor INR more frequently when
starting or stopping ciprofloxacin;
most patients will have increase in
INR, but some will experience no
effect; AMS considers empiric 10%–
15% warfarin dose reduction
Monitor INR more frequently when
starting or stopping clarithromycin;
AMS considers empiric 15%–25%
warfarin dose reduction
Monitor for bleeding
~2 hours for
3–7 days
antiplatelet (platelet aggregation
impact
is irreversibly
inhibited by
metabolite of
clopidogrel for
lifetime of the
platelet)
Delayed
NR
Monitor INR frequently when
starting or stopping cloxacillin;
(t½ = 0.5–1 hour) AMS will not empirically decrease
warfarin unless patient has other
factors affected INR (e.g., decreased
appetite, fever)
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PRACTICE TOOL Warfarin drug interactions*
Drug
Colchicine10
Danazol10,24
Direction and
severity of effect
Mechanism
on INR
↑ INR
Possibly due to diarrhea
associated with colchicine
Moderate
↑ INR
Moderate
Darunavir‡10,17,25,26
↓ INR
Moderate
Delavirdine10,14,27
↑ INR
Moderate
Diclofenac10
No effect on
INR,
­↑ risk of
bleeding
Disopyramide10
Moderate
↑ INR
Doxycycline10,28
Moderate
↑ INR
Moderate
Dronedarone29
Efavirenz10,14,30
↔ or ↑ INR
Mild
↑ or ↓ INR
Moderate to
severe
Erythromycin10
↑ INR
Moderate
25.e1 Anticipated Anticipated offset
onset
(t½)†
1–3 days
(t½ = 26.6–31.2
hours)
Decrease in warfarin
metabolism; may relate
to direct inhibition of
fibrinolytic systems
Induction of warfarin
metabolism observed
with use of ritonavir
(through CYP2C9,
CYP1A2 induction); AUC
for S-warfarin decreased
by 21% when given with
darunavir / ritonavir
combination. Induction
of warfarin metabolism
likely due to ritonavir
(through CYP2C9, CYP1A2
induction)
Possible inhibition of
warfarin metabolism
(through CYP3A4
inhibition)
Inhibition of platelets
and gastroprotective
prostaglandins
3–7 days
Unknown
2–5 days
Unknown; possible
inhibition of CYP3A4mediated warfarin
metabolism and/or proteinbinding displacement
Dronedarone 600 mg bid
increased S-warfarin 1.2fold via moderate inhibition
of CYP3A4, INR increased
1.07-fold
2–5 days
Inhibition or induction
of warfarin metabolism
(efavirenz induces CYP3A4
and may inhibit CYP2C9)
2–3 weeks
Decrease in warfarin
metabolism (through
CYP3A4 inhibition)
1–3 days
Delayed
(t½ = 24 h)
1 week
Delayed
(t½ = ~15 hours)
Delayed
Several days
(t½ = ~ 6 hours)
2–5 days
3–7 days
(t½ = 2 hours)
3-5 days
3–5 days
2–5 days
Suggested management
If patient is experiencing significant
diarrhea with colchicine (>3–4 loose
stools per day), check INR; decrease
in warfarin dose may be needed
during concurrent therapy with
colchicine
Monitor INR when starting or
stopping danazol; warfarin dose
reductions of ~50% may be
necessary
Monitor INR more frequently when
starting or stopping darunavir;
warfarin dose increase of up to
20% may be required; inductive
effect on warfarin may be due to
coadministration of ritonavir
Monitor INR more frequently when
starting or stopping delavirdine;
decrease in warfarin dosage may be
required
Minimal interaction if diclofenac
administered topically;
minimize oral use; watch for
bleeding, especially gastrointestinal
bleeding
Monitor INR when starting or
stopping disopyramide
(t½ = 4–10 hours)
NR
Monitor INR when starting or
stopping doxycycline; AMS will not
(t½ = 15–24 hours) empirically decrease warfarin unless
patient has other factors affecting
INR (e.g., decreased appetite, fever)
NR
No evidence of safety concerns with
coadministration in clinical trials
(t½ = 25–30
hours; completely
eliminated after 2
weeks)
Several weeks
Consider empiric reduction of
warfarin dose; monitor INR more
(t½ = 40–55 hours) frequently when starting or stopping
efavirenz; one patient required 4-fold
reduction in warfarin dose30
3–5 days
Monitor INR when starting or
stopping erythromycin; AMS
(t½ = ~1.5 hours) considers empiric 10%–15%
warfarin dose reduction
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PRACTICE TOOL Warfarin drug interactions*
Drug
Ethinyl estradiol10
Etravirine14,17,31
Fenofibrate10,32
Direction and
severity of effect
Mechanism
on INR
↑ or ↓ INR
Unknown; case reports of
↑ or ↓
substantial increase in INR
anticoagulation after administration of
emergency contraceptive
pill; long-term estrogen
therapy thought to be
Moderate
thrombogenic
↑ INR
Inhibition of warfarin
metabolism (through
Moderate
CYP2C9 inhibition)
↑ INR
Anticipated Anticipated offset
onset
(t½)†
2–7 days
1–2 weeks
Unknown
5–10 days
Inhibition of warfarin
metabolism (via CYP2C9and CYP3A4)
2–3 days
Inhibition of warfarin
metabolism (via CYP2C9)
1–3 weeks
Major
Fluconazole8,10,33
↑ INR
Moderate
Fluvastatin10,34
↑ INR
Moderate
Fosamprenavir‡10,13,14,17
↑ INR
Moderate
Gemfibrozil10,35,36
↑ INR
Moderate
Glyburide10
Ibuprofen10
↑ INR
Possible inhibition of
warfarin metabolism
(through CYP3A4
inhibition)
Delayed
Inhibition of warfarin
metabolism (via CYP2C9);
displacement of warfarin
from plasma-protein
binding sites
Unknown
5–7 days
Moderate
No effect, ↑ risk Inhibition of
of bleeding
functioning of platelets
and gastroprotective
prostaglandins
Delayed
~2–5 days
Suggested management
Delayed
Avoid concurrent use if possible;
monitor INR closely; monitor
(t½ = 13–27 hours) clinically for signs of bruising or
bleeding
1–2 weeks
Consider empiric reduction of
warfarin dose; monitor INR more
(t½ = 41 hours) frequently when starting or stopping
etravirine
Delayed
Monitor INR closely (i.e., weekly)
when starting or stopping
(t½ = 20–22 hours) fenofibrate; AMS considers initial
empiric 10%–15% warfarin dose
reduction, in anticipation of eventual
reduction of up to 40%
Monitor INR closely when starting
7–10 days
or stopping fluconazole; effects
(t½ = ~30 hours; more pronounced in patients with
prolonged in elderly reduced renal function due to
patients)
reduced clearance of fluconazole;
AMS considers empiric 25%–30%
warfarin dose reduction, with
eventual reductions approaching
80%
Delayed
Monitor INR when starting or
stopping fluvastatin; consider
(t½ = 2.5 h)
alternate statin (interactions
involving pravastatin and
atorvastatin have not been reported)
Several days to
Monitor INR more frequently when
weeks
starting or stopping fosamprenavir;
addition of ritonavir booster
(t½ = ~ 7.7 hours) (CYP2C9, CYP1A2 inducer) may
result in net reduction in INR; see
entry for ritonavir for additional
information
Delayed
Monitor INR when starting or
stopping gemfibrozil; consider
(t½ = 1.3 h)
empiric 10%–30% warfarin dose
reduction, with ongoing monitoring
(based on published case reports)35
Delayed
Monitor INR closely when starting
or stopping glyburide
(t½ = 5–10 hours)
3–7 days
Monitor for bleeding (especially
gastrointestinal); minimize or avoid
(t½ =1.8–2.4 hours) concurrent use of ibuprofen; take
with food
Moderate
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PRACTICE TOOL Warfarin drug interactions*
Drug
Indinavir‡10,14,17,37
Indomethacin10
Isoniazid10,21
Isotretinoin10,38
Direction and
severity of effect
Mechanism
on INR
↑ or ↓ INR
Inhibition of warfarin
metabolism (through
Moderate
CYP3A4 inhibition)
Potential ↑ INR, Inhibition of platelet
aggregation and
↑ risk of
gastroprotective
bleeding
prostaglandins
Moderate
↑ INR
Inhibition of warfarin
metabolism (via CYP2C9)
Moderate
↓ INR
Possible CYP-enzyme
induction
Anticipated Anticipated offset
onset
(t½)†
Several
weeks
2–5 days
3–5 days
Moderate
Itraconazole and
ketoconazole10
↑ INR
Moderate
Lactulose10
Lansoprazole10
↑ INR
Moderate
↑ INR
↑ INR
↑ INR
Moderate
Levothyroxine10
↑ INR
Moderate
25.e3 Delayed
Unclear
Inhibition of warfarin
metabolism (via CYP2C9
and CYP3A4)
2–5 days
Decreased intestinal
absorption of vitamin K
1–3 days
Unknown
2–7 days
NR
2–10 days
(t½ = 0.9–1.5
hours)
Delayed
Inhibition of warfarin
metabolism (via CYP2C9)
Major
Levofloxacin10,21,40,41
Monitor INR more frequently when
starting or stopping indinavir;
(t½ =1.4–2.2 hours) paradoxical case report described
unboosted indinavir leading to
decrease in INR, which required 50%
increase in warfarin dose37; addition
of ritonavir booster (CYP2C9,
CYP1A2 inducer) may result in
net reduction in INR; see entry for
ritonavir for additional information
3–7 days
Monitor INR with concomitant use;
monitor for bleeding (especially
(t½ = 4.5 hours) gastrointestinal); minimize use; take
with food
(t½ = 10–20 hours)
Moderate
Leflunomide10,39
1 week
(t½ = ~1–4 hours)
Unclear
Suggested management
3–14 days
Monitor INR when starting or
stopping isoniazid; consider empiric
10%–15% warfarin dose reduction
initially, then further reductions
based on close monitoring of INR (at
least weekly)
Monitor INR when starting or
stopping isotretinoin; case reports
indicate that increase in warfarin
dose of 33%–50% may be required38
Monitor INR closely when
starting or stopping itraconazole
or ketoconazole; AMS considers
empiric 25%–30% warfarin dose
reductions
(itraconazole t½ =
64 ± 32 hours;
ketoconazole t½ =
2–12 hours)
Delayed
Monitor INR closely when starting
or stopping lactulose
Monitor INR when starting or
stopping lansoprazole; consider reassessing INR in 1 week
Monitor INR closely when starting
or stopping leflunomide
(t½ = ~2 weeks)
Unknown; possible CYP1A2 3–5 days
inhibition; clinically
significant interaction more
common among elderly
patients
1–2 weeks
Patients with
hypothyroidism have
higher requirements
for warfarin because of
decreased catabolism of
clotting factors; correcting
hypothyroidism therefore
decreases warfarin
requirements
5–10 days
1–2 weeks
(t½ = 6–7 days)
Monitor INR closely when starting
or stopping levofloxacin; INR will be
affected by severity of illness; AMS
considers empiric 0%–15% warfarin
dose reduction
Monitor INR closely (every 1–2
weeks) when starting or adjusting
levothyroxine; adjust warfarin
gradually according to INR results
C P J / R P C • JA N UA RY / F E B RUA RY 2 0 1 1 • VO L 1 4 4 , N O 1
PRACTICE TOOL Warfarin drug interactions*
Drug
Lopinavir/ritonavir
(Kaletra)‡10,14,30,37,42,43
Mesalamine10,44
Direction and
severity of effect
Mechanism
on INR
↓ INR
Increase in warfarin
metabolism (through
Moderate
CYP2C9, CYP1A2
induction)
↓ INR
Anticipated Anticipated offset
onset
(t½)†
Several days
to weeks
(t½ = 5–6 hours)
Unknown
Delayed
↓ INR
Moderate
Methyl salicylate
(topical)10,45
Metronidazole10
↑ INR,
↑ risk of
bleeding
Moderate
↑ INR
Major
↑ INR
Miconazole (oral,
topical, or vaginal
formulation)10,46,47
Moderate
Moxifloxacin10,21,48
↑ INR
Major
Naproxen10
Nelfinavir10,14,43,49
No effect on
INR,
­↑ risk of
bleeding
Major
↑ or ↓ INR
Moderate
Nevirapine10,14,17,43,50,51
↓ INR
Moderate to
severe
NR
(t½ = 0.6–1.4
hours)
Mild to
moderate
Methimazole10
Several days to
weeks
Increased catabolism
of clotting factors
with introduction of
methimazole and return
of euthyroidism increases
warfarin requirements
Inhibition of warfarin
metabolism and platelet
aggregation
3–10 days
Delayed
NR
Decrease in warfarin
metabolism (through
CYP2C9 inhibition)
3–5 days
~ 2 days
Inhibition of warfarin
metabolism (via CYP2C9
and CYP3A4)
2–5 days
Unknown; possible
inhibition of CYP1A2;
clinically significant
interaction more common
among elderly patients
Inhibition of platelet
aggregation and production
of gastroprotective
prostaglandins
2–5 days
Possible reduction or
increase in warfarin
metabolism possible
(through CYP3A4
inhibition, CYP2C9
induction)
Increase in warfarin
metabolism (through
CYP3A4 induction)
1–2 weeks
Suggested management
Monitor INR more frequently when
starting or stopping lopinavir–
ritonavir; at steady-state, induction
interaction more likely to prevail,
resulting in reduced INR, requiring
up to a 2-fold warfarin dose increase;
see also entry for ritonavir
Monitor INR when starting or
stopping mesalamine; one patient
experienced dramatic decline in INR
and deep vein thrombosis, but INR
became therapeutic once mesalamine
was stopped44
Monitor INR closely when starting,
stopping, or adjusting methimazole
(t½ = 2–3 hours)
2–5 days
Several days
Monitor INR closely; consider
alternative therapy (topical capsaicin
is preferred alternative)
Monitor INR closely when starting
or stopping metronidazole; AMS
(t½ = 8 hours)
considers empiric 25% –40%
warfarin dose reduction
2–5 days
Monitor INR closely when starting
or stopping topical, vaginal, or oral
(t½ = 24 hours) miconazole; consider alternative
therapy (e.g., clotrimazole, which
has no interaction with warfarin);
AMS considers empiric 25%–30%
warfarin dose reduction
2–3 days
Monitor INR closely when starting
or stopping moxifloxacin; INR will
(t½ = ~12.7 hours) be affected by severity of illness; AMS
considers 0%–25% warfarin dose
reduction
3–7 days
Monitor closely for bleeding
(especially gastrointestinal); avoid or
(t½ = 12–15 hours) minimize concurrent use; take with
food
Several days
Monitor INR frequently when
starting or stopping nelfinavir
(t½ = 3.5–5 hours)
Several days
to weeks
C P J / R P C • JA N UA RY / F E B RUA RY 2 0 1 1 • VO L 1 4 4 , N O 1 Several days to
weeks
(t½ = 45 hours)
Monitor INR more frequently when
starting or stopping nevirapine;
2- to 4-fold increase in warfarin
dose may be required (based on case
reports)43,50,51
25.e4
PRACTICE TOOL Warfarin drug interactions*
Drug
Omeprazole10
Orlistat10
Phenobarbital10,36
Phenytoin10,36,52
Prednisone10
Propafenone10,53
Propoxyphene10
Propylthiouracil10
Quetiapine10,54
Raloxifene10
25.e5 Direction and
Anticipated Anticipated offset
severity of effect
Mechanism
Suggested management
onset
(t½)†
on INR
3–5 days
NR
Decrease in warfarin
↑ INR
Interaction of doubtful clinical
metabolism through
significance; minimal effect on INR;
(t½ = 0.5–1 hour) no empiric warfarin dose adjustment
Mild to
stereoselective inhibition of
moderate
the hepatic metabolism of
required
the less potent (R)-warfarin
enantiomer
↑ INR
Decreased absorption
Unknown
NR
Monitor INR closely with
of fat-soluble vitamins,
concomitant use; avoid concomitant
Moderate
including vitamin K
use if possible
↓ INR
Induction of hepatic
Delayed
NR
Monitor INR closely, especially when
metabolism of warfarin
starting or stopping phenobarbital;
Moderate
(t½ = 1.5–4.9 days) according to published reports,
30%–60% warfarin dose increases
may be required after barbiturate
initiation11,36
10–14 days
Initial: 1–3
Initially, displacement of
Initially,
Monitor INR closely when starting
days
transient ↑ in warfarin from proteinor stopping phenytoin;
(t½ = 22 hours) AMS recommends no empiric
risk of bleeding; binding sites; with longSubsequent:
with long-term term use, induction of
dose adjustment when phenytoin
2–4 weeks
hepatic metabolism of
use, ↓ INR
is initiated, but monitoring of
warfarin
INR at least weekly; some patients
Moderate
may require up to 50% warfarin
dose increase several weeks after
phenytoin is initiated; warfarin also
affects phenytoin concentration
↑ or ↓ INR
Unknown
Delayed
NR
Monitor INR when starting
or stopping prednisone; AMS
Mild
(t½ = 2.6–3 hours) recommends no empiric dose
adjustment when initiating
prednisone; warfarin dose
adjustment may be required for
patients receiving large bolus or
pulse doses of steroids; monitor for
bleeding
2–5 days
~2 days
↑ INR
Decrease in warfarin
Monitor INR when starting or
metabolism; 39% increase
stopping propafenone; AMS
(t½ = 2–10 h)
Moderate
in plasma concentration of
empirically reduces warfarin dose by
warfarin reported53
15%–30% and monitors closely, with
further reductions as required
Delayed
NR
↑ INR
Unknown (may be due to
Monitor INR when starting or
propoxyphene alone or to
stopping propoxyphene (based on
(t½ = 2.6–3 hours) published case reports only)11
Moderate
acetaminophen component
when used in combination)
Within 2
1–2 weeks
↓ INR
Increased catabolism
Monitor INR closely when starting,
weeks
of clotting factors
stopping, or adjusting dose of
(t½ = 1.5–5 hours) propylthiouracil
Moderate
with introduction of
propylthiouracil and return
of euthyroidism increases
requirement for warfarin
↑ INR
Competitive inhibition of
7–14 days
NR
Monitor INR when starting or
CYP3A4 and CYP2C9
stopping quetiapine (based on single
Moderate
(t½ = ~6 h)
case report only)54
Unknown
Rapid
NR
↓ INR
Monitor INR closely when starting
or stopping raloxifene
(t½ = 27 hours)
Moderate (based
on single-dose
studies only; no
data on longterm use)
C P J / R P C • JA N UA RY / F E B RUA RY 2 0 1 1 • VO L 1 4 4 , N O 1
PRACTICE TOOL Warfarin drug interactions*
Drug
Ranitidine10
Ribavirin55
Direction and
severity of effect
Mechanism
on INR
↑ INR
Inhibition of hepatic
metabolism of warfarin
Moderate
↓ INR
Anticipated Anticipated offset
onset
(t½)†
1–2 weeks
Unknown
2–4 weeks
Induction of hepatic
metabolism of warfarin
1–3 weeks
Moderate
Rifampin10,56
↓ INR
Moderate to
severe
Ritonavir‡10,14,17,37,49,57,58
↑ or ↓ INR
Moderate
Ropinirole10
↑ INR
Severe
Rosuvastatin10,59
↑ INR
Induction of warfarin
metabolism (through
CYP2C9, CYP1A2
induction)
Several days
to weeks
Competitive inhibition
of CYP1A2-mediated
warfarin metabolism and/
or displacement of warfarin
from binding sites
Unknown
5–10 days
3–7 days
Moderate
Saquinavir‡10,14,60
↑ INR
Moderate
Simvastatin10,34,61
↑ INR
3–7 days
Monitor INR when starting or
stopping ranitidine; consider using
(t½ = 1.9–3 hours) famotidine or nizatidine instead of
ranitidine
2–4 weeks
Monitor INR frequently when
initiating or discontinuing ribavirin
(t½ = 298 hours)
in patients taking warfarin until INR
stabilizes (~4 weeks)
1–5 weeks
Monitor INR carefully (at least
weekly) when starting or stopping
(t½ = 1.5–5 hours) rifampin; AMS considers empiric
25%–50% warfarin dose increase
initially, with further increases based
on frequent monitoring of INR (at
least weekly); patients may require
2–3 times their regular weekly
warfarin dose when rifampin is
added
1 week
Monitor INR more frequently
when starting or stopping ritonavir;
(t½ = 3–5 hours) up to 2-fold increase in warfarin
dose37,57 and 3-fold increase in
acenocoumarol dose58 documented
in case reports; another case report
documented the opposite effect
(increased INR requiring vitamin K
and decrease in warfarin dose);49 see
also entry for lopinavir–ritonavir
NR
Monitor INR closely when starting
or stopping ropinirole (based on
(t½ = 6 hours)
single published case report)10
3–7 days
(t½ = 19 hours)
Decrease in warfarin
metabolism (through
CYP3A4 inhibition)
Up to 4–8
weeks
Competition for CYP3A4mediated metabolism
3–7 days
Mild to
moderate
C P J / R P C • JA N UA RY / F E B RUA RY 2 0 1 1 • VO L 1 4 4 , N O 1 Suggested management
3–7 days
(t½ = 13 hours)
3–7 days
(t½ = 3 hours)
Monitor INR when starting or
stopping rosuvastatin; consider
alternative statin (no reports of
interaction with warfarin for
atorvastatin or pravastatin); AMS
empirically reduces warfarin dose by
10%–25% and reassesses INR within
1 week
Consider empiric decrease in
warfarin dose with use of unboosted
saquinavir; monitor INR more
frequently when starting or stopping
saquinavir; one patient required a
20% decrease in warfarin dose with
unboosted saquinavir;60 addition of
ritonavir booster (CYP2C9, CYP1A2
inducer) may result in net decrease
in INR; see entry for ritonavir for
additional information
Monitor INR when starting or
stopping simvastatin; interaction
may range from negligible to
clinically significant; consider using
alternative statin (atorvastatin or
pravastatin)
25.e6
PRACTICE TOOL Warfarin drug interactions*
Drug
Sulfamethoxazole10,62,63
(with or without
trimethoprim)
Sulfasalazine64
Direction and
severity of effect
Mechanism
on INR
↑ INR
Inhibition of warfarin
metabolism and
Severe
displacement of warfarin
from protein-binding sites
↓ INR
Unknown
Anticipated Anticipated offset
onset
(t½)†
2–5 days
Unknown
Moderate
Sulfinpyrazone10
↑ INR
Moderate
Terbinafine10
Tetracycline10
Ticlopidine10
Tipranavir‡10,14,17,50,65
Tramadol10,66
Both ↑ and ↓ Unknown
INR have been
reported
Moderate
↑ INR
Reduced plasma
prothrombin activity
Moderate
↑ INR, ↑ risk of Inhibition of metabolism
of R-warfarin (minimal
bleeding
increase in INR); decreased
Moderate
platelet aggregation
↔ or ↓ INR Possible increase in warfarin
metabolism (through
Mild
CYP3A4 induction);
however manufacturer
predicts ↔ on S-warfarin
concentration
↑ INR
Moderate
Voriconazole10
Inhibition of warfarin
metabolism (primarily
S-isomer)
↑ INR
Major
Delayed
Unknown
2–14 days
Monitor INR closely when starting
or stopping sulfamethoxazole(t½ = 10 hours) containing drug regimens; AMS
considers empiric 25%–40%
warfarin dose reduction
NR
Monitor INR frequently when
starting or stopping sulfasalazine;
(t½ = ~7.6 hours) one patient required 250% increase
in weekly warfarin dose when
sulfasalazine was started64
1–2 weeks
Monitor INR when starting or
stopping sulfinpyrazone; average
(t½ = 4–4.3 hours) daily warfarin dose decreased by
~50% in small case series11
NR
Monitor INR when starting or
stopping terbinafine
(t½ = 36 hours)
2–5 days
1–5 days
Delayed
Unknown (possible
inhibition of CYP3A4mediated warfarin
metabolism)
3–7 days
Inhibition of CYP2C9mediated metabolism of
S-warfarin
3–7 days
Suggested management
NR
Monitor INR
(t½ ~8–10 hours)
3–7 days for platelet Monitor INR when starting or
function to return stopping ticlopidine; monitor for
to baseline
increased bleeding (patient may be at
risk even if INR does not increase)
Several days to
Monitor INR more frequently when
weeks
starting or stopping tipranavir;
addition of ritonavir booster
(t½ = 5.5–6 hours) (CYP2C9, CYP1A2 inducer) may
result in net decrease in INR; use
caution when combining with
warfarin, as tipranavir has been
associated with increased risk
of intracranial hemorrhage; see
entry for ritonavir for additional
information
3–7 days
Monitor INR when starting or
stopping tramadol; dose reductions
(t½ = 5.6–6.7 hours) of 25%–30% may be required; AMS
considers empiric 0%–20% warfarin
dose reduction
NR
Monitor INR carefully when
starting or stopping voriconazole;
(t½ = 6 hours)
AMS considers empiric 25%–30%
warfarin dose reduction
AMS = Anticoagulant Management Service; ASA = acetylsalicylic acid; AUC = area under the curve; CYP = cytochrome P450; H2RA = histamine2
receptor antagonist; INR = international normalized ratio; NR = not reported; PPI = proton pump inhibitor; t½ = half-life.
*AMS management is done in reviewing the patient’s current anticoagulation status in light of their overall bleed versus clot risk. Each patient
must be assessed individually. Management options, when suggested, reflect practices / options within 2 large anticoagulation clinics in Alberta.
Information in this tool should be used with clinical judgment. This tool does not contain all drug products that may interact with warfarin and
should not be used as a substitute for comprehensive references.
†Elimination half-life assumes oral dosing in an adult with normal renal and hepatic function.
‡With the exception of nelfinavir, protease inhibitor therapy includes low-dose ritonavir, which serves as a therapeutic booster to increase
concentrations of other protease inhibitors. Interpretation of interaction data should also take into account the impact of ritonavir on warfarin
metabolism (anticipate an inductive effect at steady-state, which would result in subtherapeutic INR and a need to increase the dose of warfarin).
For more information, refer to sections on lopinavir–ritonavir and ritonavir.
25.e7 C P J / R P C • JA N UA RY / F E B RUA RY 2 0 1 1 • VO L 1 4 4 , N O 1
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