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A recently published randomized, double- blind, placebo-controlled study of 36 patients with stable international normalized ratios (INRs) demonstrated a clinically significant interaction between warfarin and high-dose acetaminophen (2-4 g/day).
Patients were randomized to receive placebo, 2 g of acetaminophen per day, or 4 g of acetaminophen per day. A stable INR was defined as 2 consecutive INRs at least 3 weeks apart that were in the therapeutic range.
Of the 24 patients assigned to an acetaminophen group, 13 had elevated INRs over the course of the 4-week study period. Two patients had elevated INRs in the placebo group.
INRs returned to normal in 83% of patients when acetaminophen was discontinued.