The safety of nizatidine as treatment for active duodenal or gastric ulcer disease or as maintenance therapy following ulcer healing was assessed in 3800 nizatidine-treated individuals in clinical trials conducted in the USA and Europe. Safety parameters included physical examinations, electrocardiograms, eye examinations, serum chemistries and testosterone, hematology, and urinalyses. Adverse events were recorded without judgment of causality. Early discontinuations and adverse events, including complications of active duodenal ulcer disease, occurred more frequently in placebo-treated patients than in those given nizatidine. No differences were observed between the nizatidine and ranitidine treatment groups in regard to adverse event incidence or severity. Minimal changes in uric acid values were observed during ranitidine and nizatidine therapy. Abnormal liver function tests occurred infrequently and to an equal extent in nizatidine and placebo treatment groups. No clinically relevant differences in laboratory test results were observed between treatment groups in studies conducted in Europe.