A histamine receptor antagonist, more commonly referred to as an H2 receptor antagonist, is a class of drug that blocks the action of histamine in the stomach. It works by indirectly decreasing the production of stomach acid secreted by parietal cells in the stomach lining. It is most commonly used for gastrointestinal issues, such as peptic ulcer disease and gastroesophogeal reflux disease (GERD).
Histamine is produced in the stomach by an Enterochromaffin-like cell (ECL). The histamine then goes on to bind to the parietal cells at specialized receptor sites called H2 receptor sites. This binding stimulates the secretion of acid in the stomach. Normally the stomach acid, called gastric acid, helps break down food particles. If there is excessive gastric acid, damage can occur in the stomach as well as the esophagus, as is the case with GERD.
The production of stomach acid can be reduced by a histamine receptor antagonist. The histamine receptor antagonist binds to the H2 receptor sites on the parietal cells, and this in turn blocks the histamine from binding to the parietal cell. As a result parietal cells are not stimulated, and produce less gastric acid. Histamine receptor antagonists only partially reduce stomach acid, usually by around 50-80%.
There are several conditions a histamine receptor antagonist is used for. The most common problem is dyspepsia, usually referred to as indigestion or upset stomach. They can also be used in cases of peptic ulcer diseases, stress ulcers, and GERD, which is more commonly referred to as heartburn. The effects typically last for six to ten hours, which give them an advantage over antacids which only last one to two hours.
The available forms of histamine receptor antagonists are famotidine, cimetidine, ranitidine, and nizatidine. The first one created, cimetidine, was created in the mid 1960s by Smith, Kline & French, now known as GlaxoSmithKline. It was first sold in 1976 under the brand name Tagamet®. They are all available over-the-counter, meaning without a prescription.
Most histamine receptor antagonists side effects are uncommon and usually mild. Side effects for cimetidine are fairly rare, but there can be interactions with other medications. Ranitidine and nizatidine can also have interactions, but these are less common than with cimetidine. Even though they are available over-the-counter, it is always advisable to check with a doctor when taking other medications to avoid adverse reactions.
Although histamine receptor antagonists are still used, they have largely been overtaken by the proton pump inhibitors in popularity. The proton pump inhibitors work by blocking the production of gastric acid, but are more effective and have fewer side effects. Both are more effective than antacids, which only dissolve acid that is currently in the stomach.