Cimetidine (INN) is a histamine H₂ receptor antagonist that inhibits stomach acid production. It is largely used in the treatment of heartburn and peptic ulcers.

Cimetidine was discovered in 1971. It has been marketed by GlaxoSmithKline (which is selling the brand to Prestige Brands) under the trade name Tagamet (sometimes Tagamet HB or Tagamet HB200). Cimetidine was approved in the UK in 1976, and was approved in the US by the Food and Drug Administration for prescriptions starting January 1, 1979.