Abstract: This invention relates to an improved anti-ulcer composition for prophylaxis and treatment of stomach and duodenum ulcers, including stress ulcers. The composition provides potent anti-ulcer activity with a dramatic reduction in toxicity and harmful side effects. The composition comprises a histamine H.sub.2 antagonist based on a guanidine derivative, preferably [N-cyano-H'-methyl-N"]-2(4-methyl-imidazolyl)-methylthio-[ethylguanidine]; and at least one 4-phenyl-tetrahydroisoquinoline antidepressant, preferably the [+] isomer of 4-phenyl-2-methyl-8-amino-1,2,3,4-tetrahydroisoquinoline hydrogen maleate. The composition may also contain a spasmolytic agent, preferably 2-metoxycarbonyl-4-(beta-piperidino ethyl) -metoxycarbonyl-4-(beta-peperidinoethyl) benzophenone. The ratio of antagonist to antidepressant ranges from 2:1 to 60:1, and the ratio of their sum to the spasmolytic agent, when used, ranges from 2.2:1 to 42.5:1.