Famotidine, sold under the brand name Pepcid among others, is a histamine H2 receptor antagonist medication that decreases stomach acid production. It is used to treat peptic ulcer disease, gastroesophageal reflux disease, and Zollinger-Ellison syndrome. It is taken by mouth or by injection into a vein. It begins working within an hour. Common side effects include headache, intestinal upset, and dizziness. Serious side effects may include pneumonia and seizures. Use in pregnancy appears safe but has not been well studied while use during breastfeeding is not recommended. Famotidine was patented in 1979 and came into medical use in 1985. It is available as a generic medication. In 2020, it was the 66th most commonly prescribed medication in the United States, with more than 10million prescriptions. Medical uses Relief of heartburn, acid indigestion, and sour stomach Treatment for gastric and duodenal ulcers Treatment for pathologic gastrointestinal hypersecretory conditions such as Zollinger–Ellison syndrome and multiple endocrine adenomas Treatment for gastroesophageal reflux disease (GERD) Treatment for esophagitis Part of a multidrug regimen for Helicobacter pylori eradication, although omeprazole may be somewhat more effective. Prevention of NSAID-induced peptic ulcers. Given to surgery patients before operations to reduce the risk of aspiration pneumonitis. Pharmacokinetics Famotidine has a delayed onset of action, beginning after 90 minutes. However, famotidine has a duration of effect of at least . At its peak effect, after administration, famotidine reduces acid secretion by 7.3 mmol per 30 minutes. Side effects The most common side effects associated with famotidine use include headache, dizziness, and constipation or diarrhea. Famotidine may contribute to QT prolongation, particularly when used with other QT-elongating drugs, or in people with poor kidney function. Mechanism of action Activation of H2 receptors located on parietal cells stimulates proton pumps to secrete acid into the stomach lumen. Famotidine, an H2 antagonist, blocks the action of histamine on the parietal cells, ultimately reducing acid secretion into the stomach. Interactions Unlike cimetidine, the first H2 antagonist, famotidine has a minimal effect on the cytochrome P450 enzyme system, and does not appear to interact with as many drugs as other medications in its class. Some exceptions include antiretrovirals such as atazanavir, chemotherapeutics such as doxorubicin, and antifungal medications such as itraconazole. History Famotidine was developed by Yamanouchi Pharmaceutical Co. It was licensed in the mid-1980s by Merck & Co. and is marketed by a joint venture between Merck and Johnson & Johnson. The imidazole