Cimetidine is a competitive histamine H2-receptor antagonist that inhibits gastric acid secretion and reduces pepsin output. It was one of the first drugs discovered using a rational design method. G. J. Durant and co-workers reported its synthesis was reported in 1974. Cimetidine is the active ingredient in Tagamet and other "heartburn" drugs and has been used off-label to treat other medical conditions.
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