Motivation:

To assess concomitant use of warfarin and acetaminophen in an outpatient population of patients receiving blood-thinning drugs as a possible risk factor for the increase of International Normalised Ratio (INR).

Methods:

Retrospective cohort study. The information pertaining 1,458 patients receiving blood-thinning drugs with warfarin was analysed. The risk factor was the joint intake of warfarin and acetaminophen; follow-up time was 1 month and the final event considered was a INR increase of or higher than 0.5 units. The association of the risk factor with the final event was establish with the Relative Risk (RR) and the Population Attributable Risk (PAR).

Results:

63 patients met the inclusion criteria, of whom 21 were exposed patients and 42 were unexposed patients. In the exposed patients a statistically significant increase of INR on the month following the onset of treatment with acetaminophen was observed, compared to the baseline values (month 1: median 3.06 vs. month 0: median 2.63), (p = 0.003). In contrast, no statistically significant differences of INR values were observed in unexposed individuals (month 0: median 2.63 vs. month 1: median 2.75) (p = 0.115). The combination of warfarin and acetaminophen represented a RR 2.5 times higher to increase INR greater than or equal to 0.5 units.

Conclusion:

Concomitant use of warfarin and acetaminophen is associated to an increase of INR equal to or greater than 0.5 units, which could generate a higher blood-thinning effect and, thus, cause a potentially higher risk of bleeding.

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