Famotidine is a histamine H2 receptor antagonist. It is selective for H2 over H1 and muscarinic receptors (Kis = 4 and 28 µM, respectively, in bovine cerebral cortex). Famotidine blocks the action of histamine in the parietal cells, ultimately blocking acid secretion in the stomach. It inhibits histamine-induced acid secretion in isolated canine parietal cells (IC50 = 0.6µM) and suppresses histamine-induced gastric acid secretion in dogs when administered orally and in anesthetized rats when administered intraduodenally (ID50s = 10 and 400µg/kg, respectively). Famotidine is used to treat ulcers, gastroesophageal reflux disease (GERD) and heartburn and it decreases the risk of gastrointestinal toxicity associated with non-steroidal anti-inflammatory drugs (NSAIDs). Potential inhibitor of the viral enzyme papain-like protease (PLpro), important for SARS-CoV-2 replication and being tested in COVID-19 clinical trials.