Famotidine

H2 antihistamine · also known as Pepcid

H2 antihistamine; blocks the histamine H2 receptor, the principal mediator of gastric acid secretion.

How it relates to mast cells: Downstream of the mast cell
Types of studies cited: Randomised controlled trial
Approved in the US for: heartburn from acid indigestion, duodenal and gastric ulcer, gastro-oesophageal reflux disease and erosive oesophagitis, pathological hypersecretory conditions
Randomised trials in: acute urticaria
Also covers: Cimetidine (Tagamet), Nizatidine (Axid)

What this relationship means: Acts after mast-cell mediators are released rather than stabilising the mast cell itself. This includes treatments that block mediator receptors, degrade a mediator, or oppose the physiology of an acute reaction. It describes where the intervention acts, not its clinical importance. A 2025 practical-management review places this at step 2 of the sequence it describes — described as add-on therapy where episodes involve gastrointestinal symptoms. That is what the review reports, not a recommendation from this site.

Mechanism of action

Famotidine antagonises the histamine H2 receptor. H2 receptors are expressed on gastric parietal cells, where they drive acid secretion, and also on vascular smooth muscle and on several immune cell populations — which is why H2 blockade is discussed in mast cell disease as covering histamine effects that H1 blockade leaves untouched.

In the stepwise mediator-blockade approach, H2 antihistamines are the conventional second step after H1 antihistamines.

Sources

Famotidine in the treatment of acute urticaria Peer-reviewed study 2000 PMID 10844490 · link checked 22 Aug 2026
Famotidine US over-the-counter Drug Facts label Drug label · link checked 22 Aug 2026
Famotidine tablets US prescribing information Drug label · link checked 21 Aug 2026
Mast Cell Activation Syndrome and Mastocytosis: Initial Treatment Options and Long-Term Management Review article 2019 PMID 30961835 · link checked 20 Aug 2026
Mast Cell Activation Syndrome Review article 2018 PMID 25944644 · link checked 20 Aug 2026

What this evidence cannot establish

Nothing controlled in MCAS. There is one small randomised double-blind trial in acute urticaria — 25 patients, an active comparator rather than placebo — which is a related condition rather than this one, and too small to carry much weight. Beyond that the entry rests on a published receptor mechanism and on expert-consensus management reviews. H2 blockade is also almost always added on top of H1 blockade, so even uncontrolled reports cannot attribute anything to the H2 agent specifically.

What this page does not say

No dosing, and no claim that this works. The entry describes a published mechanism, the cited study types, and what those studies cannot establish — nothing further. Why the site is built this way.

Recently verified. Verified 22 Aug 2026