Drug Interaction Report

MONITOR: Theoretically, anticholinergic agents and other agents with significant anticholinergic activity (e.g., antihistamines, antispasmodics, neuroleptics, phenothiazines, skeletal muscle relaxants, tricyclic antidepressants, class IA antiarrhythmics especially disopyramide) may antagonize the effects of topically administered cholinergic agents such as acetylcholine, carbachol, demecarium, echothiophate, isoflurophate, physostigmine, and pilocarpine. The proposed mechanism involves opposing pharmacodynamic action on muscarinic receptor sites in ocular tissue. This interaction is sometimes desirable and is the basis for using atropine in the treatment of excessive muscarinic side effects and cholinergic crisis induced by cholinergic overdose.

MANAGEMENT: Patients receiving long-term therapy with anticholinergic agents should be monitored for potentially diminished therapeutic (miotic) response to ophthalmic cholinergic therapy, and dosages adjusted as necessary.

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.