Two commonly used anti-ulcer drugs increase blood-alcohol concentrations in patients who drink socially, according to an American Medical Assn. survey. The study said a warning may be appropriate when Glaxo's Zantac (ranitidine) and SmithKline's Tagamet (cimetidine) drugs are prescribed, especially for social drinkers who may drive vehicles or operate machinery. Physicians usually advise ulcer patients not to drink alcoholic beverages. Zantac increased the concentrations by 34%, while Tagamet increased them by 92%, said the study published in the Journal of the American Medical Assn.