H2 antagonist
An H2 antagonist turns down the stomach's acid by silencing one of the signals that tells it to switch on. Histamine is one of the chemical 'go' messages for acid secretion; an H2 blocker sits on the histamine H2 receptor so that message is not heard.
Drugs such as famotidine, ranitidine (now largely withdrawn over an impurity concern) and cimetidine competitively block H2 receptors on the gastric parietal cell. Histamine normally acts through these receptors to raise cyclic AMP and drive the proton pump, so blocking them reduces both basal and stimulated acid output. They act more slowly than antacids but last longer, and they are weaker acid suppressors than proton-pump inhibitors.
They are useful for milder reflux and ulcer symptoms and for night-time acid control. Tolerance can develop with continuous use as the body up-regulates receptors. Cimetidine is the most interaction-prone of the group because it inhibits several cytochrome P450 enzymes; newer agents such as famotidine have far fewer interactions and are generally preferred.
Someone with occasional night-time reflux takes famotidine at bedtime to keep acid output low while sleeping.
Famotidine blocks the parietal-cell H2 receptor to reduce histamine-driven acid secretion.
The H2 receptor here is unrelated to the H1 receptor blocked by allergy antihistamines; both are histamine receptors but with different jobs and different drugs.