RT Journal A1 Gray CD T1 ACetaminophen and risk factors for excess anticoagulation with warfarin JF JAMA JO JAMA YR 1998 FD August 26 VO 280 IS 8 SP 695 OP 697 DO 10-1001/pubs.JAMA-ISSN-0098-7484-280-8-jbk0826 UL http://dx.doi.org/10-1001/pubs.JAMA-ISSN-0098-7484-280-8-jbk0826 AB To the Editor.—Although the article by Dr Hylek et al1 brought to light an often-overlooked potential drug interaction between acetaminophen and warfarin, the authors' conclusions deserve comment. First, it is erroneous to make conclusions regarding cause and effect based on data obtained in a retrospective study. The study by Hylek et al indicates that there is an association between acetaminophen intake and warfarin's effect, yet the possibility of another undefined confounding factor cannot be fully dismissed. Numerous factors in the current trial were independently associated with the increased likelihood of having an INR greater than 6. It seems reasonable and quite likely that overlap could have occurred between those factors and acetaminophen intake. For example, a patient with a febrile illness may take acetaminophen to reduce fever and may have decreased oral intake.