Q: Which most commonly unpredicted drug interaction of warfarin may go unnoticed and may even without a glance of physicians in ICU, inpatient and outpatient settings?


Answer:  Warfarin and acetaminophen

INR may go dangerously high if a patient use or get prescribe 3 or 4 extra-strength acetaminophen per day, which is a common practice in ICU and other health care settings. Also to add, it is the most commonly used over the counter medicine. This interaction has no linear association and can be very unpredictable. 


For further reading/list of references:

Renato D. Lopes, John D. Horowitz, David A. Garcia, Mark A. Crowther, Elaine M. Hylek - Warfarin and acetaminophen interaction: a summary of the evidence and biologic plausibility - Blood 2011 118:6269-6273