Orally Administered Drugs
Potential pharmacokinetic interaction (altered GI absorption of various drugs): Antimuscarinics may inhibit GI motility, delay gastric emptying, and prolong GI transit time. 
Specific Drugs
Drug
Interaction
Comments
Amantadine
Possible additive anticholinergic adverse effects 
Use concomitantly with caution 
Antacids
Possible decreased absorption of antimuscarinic 
Administer oral scopolamine at least 1 hour before antacids
Antiarrhythmic agents (quinidine, disopyramide, procainamide)
Possible additive anticholinergic adverse effects 
Use concomitantly with caution 
Antidepressants, tricyclic
Possible additive anticholinergic adverse effects 
Use concomitantly with caution 
Antihistamine agents (meclizine)
Possible additive anticholinergic adverse effects 
Use concomitantly with caution 
Antiparkinsonian agents
Possible additive anticholinergic adverse effects 
Use concomitantly with caution 
Belladonna alkaloids
Possible additive anticholinergic effects 
Use concomitantly with extra caution 
Corticosteroids
Possible increased intraocular pressure
CNS depressants (e.g., sedatives, tranquilizers, alcohol)
Possible increased CNS depressive effects   (see CNS Effects under Cautions.)
Use concomitantly with caution   
Glutethimide
Possible additive anticholinergic adverse effects
Use concomitantly with caution 
Ketoconazole
Possible decreased ketoconazole absorption
If concomitant therapy is necessary, give antimuscarinic at least 2 hours after ketoconazole
Levodopa
Possible increased gastric metabolism of levodopa and decreased levodopa absorption in the small intestine
Toxicity may result from increased levodopa absorption if antimuscarinic is discontinued without a concomitant reduction in levodopa dosage
Meperidine
Possible additive anticholinergic adverse effects 
Use concomitantly with caution 
Phenothiazines
Possible additive anticholinergic adverse effects 
Use concomitantly with caution 
Potassium chloride
Antimuscarinics may potentiate potassium chloride's local GI mucosal effects
Use antimuscarinics cautiously with potassium chloride preparations (especially wax-matrix preparations), monitor carefully for evidence of GI mucosal lesions
Skeletal muscle relaxants
Possible additive anticholinergic adverse effects 
Use concomitantly with caution 
 Scopolamine drug interactions (more detail)
