Cookies on this website
We use cookies to ensure that we give you the best experience on our website. If you click 'Continue' we'll assume that you are happy to receive all cookies and you won't see this message again. Click 'Find out more' for information on how to change your cookie settings.

IntroductionCytochrome P450 (CYP) plays a key role in the metabolism of coumarin anticoagulants and nonsteroidal anti-inflammatory drugs (NSAIDs). Because CYP2C9 is a genetically polymorphic enzyme, genetic variability could play an important role in the potential interaction between NSAIDs and coumarins. We investigated whether NSAIDs were associated with overanticoagulation during therapy with coumarins and evaluated the effect of the CYP2C9 polymorphisms on this potential interaction.MethodsWe conducted a population-based cohort study among patients of an anticoagulation clinic who were treated with acenocoumarol or phenprocoumon between April 1, 1991, and May 31, 2003, and whose CYP2C9 status was known. Patients were followed up until an international normalized ratio (INR) of 6.0 or greater was reached or until the end of treatment, death, or the end of the study. Proportional hazards regression analysis was used to estimate the risk of an INR of 6.0 or greater in relation to concomitant use of a coumarin anticoagulant and NSAIDs after adjustment for several potentially confounding factors. To study effect modification by CYP2C9 genotype, stratified analyses were performed for wild-type patients and patients with a variant genotype.ResultsOf the 973 patients in the cohort, 415 had an INR of 6.0 or greater. Several NSAIDs increased the risk of overanticoagulation. The risk of overanticoagulation was 2.98 (95% confidence interval, 1.09-7.02) in coumarin-treated patients taking NSAIDs with a CYP2C9*2 allele and 10.8 (95% confidence interval, 2.57-34.6) in those with a CYP2C9*3 allele.ConclusionsSeveral NSAIDs were associated with overanticoagulation. For NSAIDs that are known CYP2C9 substrates, this risk was modified by allelic variants of CYP2C9. More frequent INR monitoring of patients taking NSAIDs is warranted.

Original publication




Journal article


Clinical pharmacology and therapeutics

Publication Date





479 - 485


Pharmacoepidemiology Unit, Department of Internal Medicine, Erasmus MC, Rotterdam, The Netherlands.


Humans, Acenocoumarol, Phenprocoumon, Aryl Hydrocarbon Hydroxylases, Anti-Inflammatory Agents, Non-Steroidal, Anticoagulants, Cohort Studies, Drug Interactions, Polymorphism, Genetic, Alleles, Aged, Aged, 80 and over, Middle Aged, Female, Male, Drug Overdose, Cytochrome P-450 CYP2C9