Download DRUGS TO TRY AND AVOID IN WARFARIN PATIENTS

COMMON WARFARIN DRUG INTERACTIONS
The drugs in this list are more usually associated with loss of INR control in patients already established on
warfarin. Patients already on any of these drugs may be started on warfarin and the dose adjusted as
required. Drug interactions with warfarin tend to be variable and unpredictable and on the whole affect a fairly
small proportion of patients.
If any of the drugs below are to be started in these patients then the use of alternatives in the same therapeutic
class may be considered. If this is not possible then the patients INR should be monitored as detailed below.
Drugs marked * are liver enzyme inhibitors and increase the INR. They act very quickly (can be within 24 hours)
and if the drug is withdrawn the effect disappears quickly depending on the drug half-life. The INR should if
possible be monitored within 72 hours of starting the interacting drug.
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Drugs marked are liver enzyme inducers and decrease the INR. They act more slowly (up to a week) with
peak effect at 2-3 weeks and can persist for up to 4 weeks after stopping depending on drug half-life. The INR
will need checking after 1 week of concurrent therapy. Drugs in bold commonly cause variations in INR.
N.B. If a patient on warfarin were started on ANY other new medication a repeat INR within 1 week
would be a sensible precaution.
Common drugs that increase the INR
Gastrointestinal
Cardiovascular
CNS
Anti-infectives
(anti-infectives in general MAY cause
raised INR’s)
Endocrine
NSAID’s
N.B. ALL NSAID’s can increase the
risk of bleeds and should be avoided
where possible – IBUPROFEN (+/- PPI)
is probably safest if an NSAID is
required
Miscellaneous
Herbal preparations etc
cimetidine*, omeprazole*
amiodarone*, fibrates, fluvastatin*, propafenone*,
propranolol, simvastatin*
dextropropoxyphene* (in co-proxamol), fluvoxamine*,
paracetamol (prolonged use at high dose), SSRI’s*, tramadol
azole antifungals* (esp. miconazole including oral gel and
vaginal), co-trimoxazole*, isoniazid*, macrolides* (can be
serious but unpredictable), metronidazole*, quinolones* (can
be serious but unpredictable), tetracyclines
anabolic steroids (and danazol), high dose corticosteroids,
glucagon (high dose 50mg+ over 2 days), flutamide,
levothyroxine, propylthiouracil
Do NOT use azapropazone and phenylbutazone
alcohol (acute), allopurinol*, disulfiram, gemcitabine,
fluorouracil, interferon, sulfinpyrazone, tamoxifen, zafirlukast*
boldo, carnitine, cranberry juice*, devils claw, danshen, dong
quai, fenugreek, fish oils, garlic, ginger, ginko biloba,
glucosamine (+/- chondroitin), lycium*, mango, melilot, papain,
PC-SPES, quilinggao, topical methylsalicylate, sweet
woodruff, tonka
Common drugs that decrease the INR
Miscellaneous
Herbal preparation etc
Binding agents
Warfarin antagonist
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Alcohol (chronic), azathioprine, barbiturates , bosentan ,
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carbamazepine , griseofulvin , mercaptopurine, mesalmine,
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nevirapine , OCP/HRT, raloxifene, ribavarin, rifampicin
(most potent inducer), trazadone
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avocado, co-enzyme Q10, green tea, natto, St Johns wort
colestyramine, sucralfate
vitamin K

Antiplatelet drugs can increase the bleeding risk and should be used only if the benefit
outweighs the bleeding risk
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Ginseng, phenytoin and quinidine can raise or lower INR
N.B This list is not exhaustive
Warfarin interactions H+T v7 Aug 2016 Review Aug 2018