Acetaminophen and Risk Factors for Excess Anticoagulation With Warfarin

To the Editor.—Dr Hylek and colleagues overemphasized acetaminophen intake as a potential risk factor for overanticoagulation when other variables appeared to have a stronger association. Four times as many case patients had decreased dietary oral intake or a new potentiating medication and 3 times as many had acute diarrhea, whereas fewer than twice as many case patients were taking acetaminophen compared with control patients (56% vs 36%). The odds ratio for any acetaminophen use resulting in overanticoagulation was not given but was likely nonsignificant, as more than half the case patients taking acetaminophen were taking fewer than 7 tablets per week, corresponding to an odds ratio of 1.0 to 1.1 as shown in Table 2 of the article. Also, since most of the patients taking higher doses had recently started taking acetaminophen, these data cannot be extrapolated to patients taking long-term high-dose acetaminophen (3-4 g daily) for osteoarthritis.