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Acetaminophen a Risk Factor for Overanticoagulation.

Aspirin and NSAIDs are relatively contraindicated for patients who must use warfarin for long-term anticoagulation, but this case-control study suggests that the alternative analgesic acetominophen may not be completely safe.

The investigators interviewed 93 patients treated in an outpatient anticoagulant therapy unit who developed international normalized ratios (INRs) greater than 6.0 while on chronic warfarin therapy. One hundred ninety-six controls were selected from those whose INRs were between 1.7 and 3.3. Acetaminophen ingestion was independently associated with greater risk for an INR over 6.0, with adjusted odds ratios of 3.5 for those who used 7 to 13 tablets per week, 6.9 for those who used 14 to 27 tablets per week, and 10.0 for those who used 28 or more tablets per week. Other factors independently associated with an increased risk for an elevated INR were advanced malignancy (adjusted odds ratio, 16.4), newly started potentiating medication (8.5), warfarin dose more than predicted (8.1), decreased oral intake (3.6), and acute diarrheal illness (3.5). Consumption of alcohol and foods rich in vitamin K were inversely correlated with an INR greater than 6.0.

Comment: These data demonstrate that several factors can cause marked shifts in anticoagulation status, and that INRs should be monitored with caution during such exposures. How acetaminophen potentiates warfarin is not clear.

— TH Lee

Published in Journal Watch General Medicine March 13, 1998

Citation(s):

Hylek EM et al. Acetaminophen and other risk factors for excessive warfarin anticoagulation. JAMA 1998 Mar 4 279 657-662.

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