Frequency of concurrent use of warfarin with potentially interacting drugs.
OBJECTIVE
To determine the rates of concomitant use of drugs known to interact with warfarin by increasing the prothrombin time expressed as the international normalized ratio (INR), decreasing the INR, or increasing bleeding risk without apparent changes in INR in a cohort of patients receiving long-term warfarin therapy.


METHODS
Retrospective, longitudinal cohort study.


METHODS
Large pharmacy benefits manager database.


METHODS
A total of 134,833 patients who were prescribed long-term warfarin from June 1, 1999-May 31, 2000.


RESULTS
Longitudinal pharmacy claims from the pharmacy benefits manager database were reviewed to identify coprescription of warfarin and drugs associated with significant interactions with warfarin. Of the 134,833 patients receiving long-term warfarin therapy, 109,998 (81.6%) were prescribed a concurrent prescription for at least one potentially interacting drug, including 87,346 (64.8%) who were prescribed one or more concomitant drugs associated with interactions known to increase the INR. Acetaminophen-containing products, prescribed for 22.7% of patients receiving concomitant prescriptions, and thyroid hormones, prescribed for 17.5%, were the most commonly prescribed concurrent drugs associated with an increased INR response. The most frequently prescribed interacting agents associated with a decreased INR response were trazodone (2.2%) and carbamazepine (1.1%). The most commonly prescribed agents independently associated with increased bleeding risk were cyclooxygenase-2 inhibitors.


CONCLUSIONS
Many patients receiving warfarin therapy are treated with concomitant drugs that may interact with the warfarin. The high percentage of patients taking drugs that may increase INR or bleeding risk is a reminder that bleeding events are a likely adverse outcome of combining drugs that interact with warfarin. Careful warfarin management is necessary to avoid adverse events associated with drug interactions.
Wittkowsky
Ann K
AK
School of Pharmacy, University of Washington, Seattle, Washington, USA. akwitt@u.washington.edu
Boccuzzi
Stephen J
SJ
Wogen
Jenifer
J
Wygant
Gail
G
Patel
Pankaj
P
Hauch
Ole
O
D000328:Adult; D000368:Aged; D000925:Anticoagulants / Q000008:administration & dosage*; D015331:Cohort Studies; D004347:Drug Interactions; D005260:Female; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D012189:Retrospective Studies; D014859:Warfarin / Q000008:administration & dosage* / Q000009:adverse effects
D000925:Anticoagulants; D014859:Warfarin