In this double-blind, randomized study, 200 normal subjects received a three-day course of one of five treatment regimens: lincomycin hydrochloride monohydrate injection (sterile solution, 300 mg/ml) with two tablets of either placebo, a mixture of atropine sulfate and diphenoxylate hydrochloride (Lomotil), an aspirin-phenacetin-caffeine (APC) combination or the latter with codeine, or an injection of saline with two placebo tablets. Gastrointestinal irritation was most prominent in subjects receiving lincomycin with atropine-diphenoxylate and lincomycin with APC plus codeine (P <.05). Decreased intestinal motility from atropine-diphenoxylate or codeine may increase the contact time between the lincomycin (or its metabolites) or some developing toxic substances and the mucosal epithelium. The use of atropine-diphenoxylate or codeine in treating lincomycin-induced diarrhea may be questionable. (JAMA 235:1451-1454, 1976)