Drug Interaction Report

Coadministration with aluminum- or magnesium-containing antacids may decrease the serum concentrations of orally administered phenothiazines. The proposed mechanism is antacid adsorption resulting in reduced phenothiazine bioavailability. The interaction has been reported for chlorpromazine but may occur with other phenothiazines. In a study of ten patients treated with chlorpromazine, 30 mL of an antacid containing aluminum and magnesium hydroxide reduced the urinary excretion of chlorpromazine by 10% to 45%. In another study with six psychiatric patients, coadministration of chlorpromazine oral suspension with 30 mL of an antacid containing aluminum hydroxide and magnesium trisilicate resulted in a 20% reduction in serum chlorpromazine level two hours later. The clinical significance is unknown. Psychiatric relapse occurred in a chlorpromazine-treated patient following the addition of antacid therapy according to a single case report. Separating the times of administration by 2 to 3 hours may help if an interaction is suspected.

GENERALLY AVOID: Concurrent use of ethanol and phenothiazines may result in additive CNS depression and psychomotor impairment. Also, ethanol may precipitate dystonic reactions in patients who are taking phenothiazines. The two drugs probably act on different sites in the brain, although the exact mechanism of the interaction is not known.

MANAGEMENT: Patients should be advised to avoid alcohol during phenothiazine therapy.