This chapter discusses several gastrointestinal drugs and their adverse effects. Aluminum-induced osteomalacia in chronic users of aluminum-containing antacids is a well-known problem. Isolated cases continue to be reported; two reports merit comment. One of them described a case of osteomalacia secondary to ingestion of large amounts of aluminum-containing antacids with a histomorphometric analysis. Gastrointestinal Magnesium-induced diarrhea may be a difficult clinical problem. Cisapride has a similar chemical structure to metoclopramide but does not have dopamine receptor blocking activity, and therefore no central antiemetic effect. However, it stimulates the release of acetylcholine in the gastrointestinal system and appears to be as effective as metoclopramide and domperidone in the management of mild reflux esophagitis and gastroparesis. Famotidine is not more effective than cimetidine and ranitidine in ulcer healing and it has an adverse-event profile similar to these earlier compounds, as has been shown in recent clinical trials.