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'''H2 antagonists''', also called '''H2 blockers''', are a class of medications+ that block the action of histamine+ at the histamine H2 receptor+s of the parietal cell+s in the stomach+. This decreases the production of stomach acid+. H2 antagonists can be used in the treatment of dyspepsia+, but have been surpassed by the more effective proton pump inhibitor+s. They are also used to treat peptic ulcer disease+ and gastroesophageal reflux disease+.

H2 antagonists are a type of antihistamine+, although in common use the term "antihistamine" is often reserved for H1 antagonists+. Like the H1 antagonists, some H2 antagonists function as inverse agonist+s rather than receptor antagonist+s, due to the constitutive activity+ of these receptors.

The prototypical H2 antagonist, called cimetidine+, was developed by Sir James Black+ at Smith, Kline and French (now GlaxoSmithKline+) in the mid-to-late 1960s. It was first marketed in 1976 and sold under the trade name Tagamet, which became the first blockbuster drug+. The use of quantitative structure-activity relationship+s (QSAR) led to the development of other agents—starting with ranitidine+, first sold as Zantac, which has fewer adverse effects+ and drug interaction+s and is more potent.

* cimetidine+
* ranitidine+
* famotidine+
* nizatidine+

Cimetidine+ was the prototypical histamine H2-receptor antagonist from which later drugs were developed. Cimetidine was the culmination of a project at Smith, Kline and French (SK&F; now GlaxoSmithKline+) by James W. Black+, C. Robin Ganellin+, and others to develop a histamine receptor+ antagonist+ that would suppress stomach acid secretion.

In 1964 it was known that histamine+ stimulated the secretion of stomach acid, and also that traditional antihistamine+s had no effect on acid production. From these facts the SK&F scientists postulated the existence of two different types of histamine receptors. They designated the one acted upon by the traditional antihistamines as H1, and the one acted upon by histamine to stimulate the secretion of stomach acid as H2.

The SK&F team used a classical design process starting from the structure of histamine. Hundreds of modified compounds were synthesised in an effort to develop a model of the then-unknown H2 receptor. The first breakthrough was ''Nα''-guanylhistamine, a partial H2-receptor antagonist. From this lead, the receptor model was further refined, which eventually led to the development of burimamide+, a specific competitive antagonist+ at the H2 receptor. Burimamide+ is 100 times more potent than ''Nα''-guanylhistamine, proving its efficacy on the H2 receptor.

The potency of burimamide was still too low for oral administration. And efforts on further improvement of the structure, based on the structure modification in the stomach due to the acid dissociation constant+ of the compound, led to the development of metiamide+. Metiamide was an effective agent; however, it was associated with unacceptable nephrotoxicity+ and agranulocytosis+. It was proposed that the toxicity arose from the thiourea+ group, and similar guanidine+ analogues were investigated until the discovery of cimetidine+, which would become the first clinically successful H2 antagonist.

Ranitidine+ (common brand name Zantac) was developed by Glaxo (also now GlaxoSmithKline+), in an effort to match the success of Smith, Kline and French with cimetidine. Ranitidine was also the result of a rational drug design process utilising the by-then-fairly-refined model of the histamine H2 receptor and quantitative structure-activity relationships (QSAR+).

Glaxo refined the model further by replacing the imidazole+-ring of cimetidine with a furan+-ring with a nitrogen+-containing substituent, and in doing so developed ranitidine. Ranitidine was found to have a far-improved tolerability profile (i.e. fewer adverse drug reaction+s), longer-lasting action, and ten times the activity of cimetidine.

Ranitidine was introduced in 1981 and was the world's biggest-selling prescription drug by 1988. The H2-receptor antagonists have since largely been superseded by the even more effective proton pump inhibitor+s, with omeprazole+ becoming the biggest-selling drug for many years.

The H2 antagonists are competitive antagonist+s of histamine+ at the parietal cell+'s H2 receptor+. They suppress the normal secretion of acid by parietal cells and the meal-stimulated secretion of acid. They accomplish this by two mechanisms: Histamine released by ECL cell+s in the stomach is blocked from binding on parietal cell H2 receptors, which stimulate acid secretion; therefore, other substances that promote acid secretion (such as gastrin+ and acetylcholine+) have a reduced effect on parietal cells when the H2 receptors are blocked.

H2-antagonists are used by clinicians in the treatment of acid-related gastrointestinal condition+s, including:Rossi S (Ed.) (2005). ''Australian Medicines Handbook+'' 2005. Adelaide: Australian Medicines Handbook. ISBN 0-9578521-9-3

* Peptic ulcer+ disease (PUD)
* Gastroesophageal reflux disease+ (GERD/GORD)
* Dyspepsia+
* Prevention of stress ulcer+ (a specific indication of ranitidine)

People who suffer from infrequent heartburn may take either antacids+ or H2-receptor antagonists for treatment. The H2-antagonists offer several advantages over antacids, including longer duration of action (6–10 hours vs 1–2 hours for antacids), greater efficacy, and ability to be used prophylactically before meals to reduce the chance of heartburn occurring. Proton pump inhibitors+, however, are the preferred treatment for erosive esophagitis+ since they have been shown to promote healing better than H2-antagonists.

H2 antagonists are, in general, well-tolerated, except for cimetidine+, wherein all of the following adverse drug reaction+s (ADRs) are ''common''. Infrequent ADRs include hypotension+. Rare ADRs include: headache+, tiredness, dizziness, confusion, diarrhea+, constipation, and rash. In addition, gynecomastia+ occurred in 0.1% to .5% of men treated for nonhypersecretory conditions with cimetidine for 1 month or longer and in about 2% of men treated for pathologic hypersecretory conditions; in even fewer men, cimetidine may also cause loss of libido, and impotence+, all of which are reversible upon discontinuation.

A 31-study review found that overall risk of pneumonia is about 1 in 4 higher among H2 antagonist users.

With regard to pharmacokinetics+, cimetidine in particular interferes with some of the body's mechanisms of drug metabolism+ and elimination through the liver cytochrome P450+ (CYP) pathway. To be specific, cimetidine is an inhibitor of the P450 enzymes CYP1A2+, CYP2C9+, CYP2C19+, CYP2D6+, CYP2E1+, CYP3A4+. By reducing the metabolism of drugs through these enzymes, cimetidine may increase their serum+ concentration+s to toxic+ levels. Many drugs are affected, including warfarin+, theophylline+, phenytoin+, lidocaine+, quinidine+, propranolol+, labetalol+, metoprolol+, methadone+, tricyclic antidepressants+, some benzodiazepines+, dihydropyridine calcium channel blocker+s, sulfonylureas+, metronidazole+, and some recreational drugs such as ethanol+ and methylenedioxymethamphetamine (MDMA)+.

The more recently developed H2-receptor antagonists are less likely to alter CYP metabolism. Ranitidine+ is not as potent a CYP inhibitor as cimetidine, although it still shares several of the latter's interactions (such as with warfarin, theophylline, phenytoin, metoprolol, and midazolam). Famotidine+ has negligible effect on the CYP system, and appears to have no significant interactions.

* Antihistamine+

Major Drug Groups:
Drugs for peptic ulcer and GORD:

H2 antagonist+ H2 antagonists, also called H2 blockers, are a class of medications that block the action of histamine at the histamine H2 receptors of the parietal cells in the stomach.
H1 antagonist+ H1 antagonists, also called H1 blockers, are a class of medications that block the action of histamine at the H1 receptor, helping relieve allergic reactions.
 H3 receptor antagonist+ An H3 receptor antagonist is a classification of drugs used to block the action of histamine at the H3 receptor.