This question is about an impoant drug interaction occurring between erythromycin and cisapride (used for gastroesophageal reflux disease- GERD). Serious cardiac arrhythmias, including torsades de pointes, QT interval prolongation, ventricular tachycardia, and ventricular fibrillation, have been repoed in patients taking cisapride with medications that inhibit cytochrome P450 3A4. Examples of medications other than cisapride that have a similar interaction with erythromycin include ketoconazole, fluconazole, clarithromycin, nefazodone, and indinavir. It is impoant to note that QT prolongation, torsades de pointes, cardiac arrest, and sudden death have occurred in patients taking only cisapride. None of the other agents listed interact with erythromycin to produce these proarrhythmic effects. Famotidine is an H2 receptor antagonist used in the treatment of GERD and gastric ulcers; this agent is generally well tolerated with very little incidence of drug interactions. Lansoprazole is a proton pump inhibitor indicated for the treatment of gastric ulcerations and GERD. When this agent is administered with phenytoin, the clearance of phenytoin is decreased by 15%, leading to an extension of the therapeutic effect. Metoclopramide is a prokinetic agent indicated for the treatment of GERD and diabetic gastroparesis. When given with cyclosporine, the toxic effects of cyclosporine are more pronounced. Fuhermore, when metoclopramide is administered with levodopa, it decreases the effectiveness of levodopa.