Famotidine, sold under the brand name Pepcid among others, is a histamine H2 receptor antagonist medication that decreases stomach acid production.

[4] Common side effects include headache, abdominal pain, diarrhea or constipation, and dizziness.

[21][22] Famotidine may contribute to QT prolongation,[23] particularly when used with other QT-elongating drugs, or in people with poor kidney function.

[24] 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.

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.

In the United States and Canada, a product called Pepcid Complete, which combines famotidine with an antacid in a chewable tablet to relieve the symptoms of excess stomach acid quickly, is available.

[38] Famotidine primarily works by blocking the effects of histamine and has some potential mechanisms of action that may contribute to its anti-inflammatory properties, including the inhibition of the production of certain pro-inflammatory cytokines such as TNF-alpha and IL-6.

[39][40] Another hypothesis was that famotidine might activate the vagus nerve inflammatory reflex to attenuate cytokine storm.