Drug interactions.

The interaction between warfarin and thyroid replacement is one of the most frequent alerts provided to pharmacists by computerized drug interaction systems, so it is important to assess any new information on this interaction. Historically, we have held that the interaction between warfarin and thyroid would rarely be clinically important, because the patient is usually taking thyroid first, and is then titrated on warfarin to the appropriate anticoagulation level. New information suggests that even when the patient is on the warfarin first, severe hemorrhage is unlikely.