Novartis. The manufacturer of carbamazepine has alerted health professionals that carbamazepine suspension (TegretolR Suspension: Novartis) should not be administered simultaneously with other liquid medicinal agents or diluents because of a risk of precipitation and the possibility of decreased bioavailability.
The company has submitted to the US Food and Drug Administration a report of a patient who passed an orange rubbery mass in his stool the day after ingesting TegretolR Suspension immediately followed by chlorpromazine (ThorazineR) solution. Subsequent testing has shown that mixing TegretolR Suspension and chlorpromazine solution (both generic and brand name) as well as TegretolR Suspension and liquid MellarilR (thioridazine) results in the precipitation of a rubbery orange mass. It is not known if the formation of this precipitate results in decreased bioavailability of carbamazepine or of either of the other two medications tested.
The extent to which this interaction occurs, if at all, with other liquid medications is not known. Systematic testing is under way to determine the cause and nature of this interaction.
The package insert has been modified to include this information in the relevant sections.
Reference: "Dear Doctor" letter, Novartis Pharmaceutical Corporation, East Hanover, USA, March 1998.