More pronounced therapeutic & undesirable effects w/ anticholinergics. Reduced therapeutic effect w/ cholinergic receptor agonists. Reduced effect of metoclopramide & cisapride. Increased AUC w/ potent CYP3A4 inhibitors eg, ketoconazole; ritonavir, nelfinavir, itraconazole. Concomitant use w/ CYP3A4 substrates w/ higher affinity (eg, verapamil, diltiazem) & CYP3A4 inducers (eg, rifampicin, phenytoin, carbamazepine).