Drug Interaction Report

MONITOR: Use of anticholinergic drugs after administration of botulinum toxin may potentiate systemic anticholinergic effects such as dry mouth, blurred vision, and urinary disorders. Botulinum toxin inhibits the release of acetylcholine from peripheral cholinergic nerve endings, thus additive or synergistic anticholinergic effects may occur when these agents are used together.

MANAGEMENT: Patients should be advised that systemic anticholinergic side effects such as dry mouth, blurred vision, and urinary disorders may increase if agents with anticholinergic properties (e.g., sedating antihistamines; antispasmodics; neuroleptics; phenothiazines; skeletal muscle relaxants; tricyclic antidepressants; disopyramide) are used after administration of botulinum toxin.

The consumption of grapefruit juice may be associated with increased plasma concentrations of darifenacin. The mechanism is inhibition of CYP450 3A4-mediated first-pass metabolism in the gut wall by certain compounds present in grapefruits. The clinical significance is unknown.