Histamine H2-receptor antagonists, also called H2 blockers, are medications that help reduce the production of gastric acid.
Because of that, they can be used to treat peptic ulcers; gastroesophageal reflux disease, or GERD; and Zollinger-Ellison syndrome, which is a rare condition caused by gastrin-secreting tumors. In addition, histamine H2-receptor antagonists can be used to prevent stress-induced ulcers.
These medications include cimetidine, which can be administered orally, intravenously, and intramuscularly; famotidine, which is given orally or intravenously; and nizatidine, which is given orally.
Now, gastric acid is secreted by the parietal cells, which have different membrane receptors that modulate their activity. One of them is the H2 histamine receptor.
Normally, the stomach is protected from the acidic environment by the mucus secreted by foveolar cells, and the esophagus is protected by the lower esophageal sphincter, which prevents gastric acid from coming back up.
When some of these mechanisms fail, the person can develop peptic ulcers or gastroesophageal reflux disease.
Histamine H2-receptor antagonists block H2 receptors so histamine can’t bind them, which decreases gastric acid secretion, and in turn mitigates the harmful effects of gastric acid in these disorders.
Generally, histamine H2-receptor antagonists are safe medications. However, they may cause some side effects, since H2 receptors are not only located in the stomach.
So, in the brain, some side effects include headaches, dizziness, somnolence, confusion, and hallucinations.
In the skin, these medications may cause pruritus and a rash; while cardiovascular side effects include hypotension and cardiac dysrhythmias.
Histamine H2-receptor antagonists may lead to sexual side effects like decreased libido and impotence; as well as gastrointestinal side effects like diarrhea or constipation.
In addition, histamine H2-receptor antagonists reduce the production of gastric acid, which can favor bacterial growth in the stomach; this is also associated with an increased risk of microaspiration and lung colonization, leading to pneumonia.
Finally, prolonged use of histamine H2-receptor antagonists for over 2 years may lead to vitamin B12 malabsorption and deficiency.
Now, cimetidine also has additional anti-androgenic effects and could cause gynecomastia, galactorrhea, and alopecia.
On the other hand, famotidine can also cause seizures and QT prolongation in clients with renal disease, and very rarely, clients may develop hypersensitivity reactions like toxic epidermal necrolysis and Stevens-Johnson syndrome.
As far as contraindications go, histamine H2-receptor antagonists should be used with caution during pregnancy and breastfeeding.
Caution should also be taken in children under 16 years of age and elderly clients. Cimetidine and famotidine should also be used with caution in clients with hepatic disease
In addition, cimetidine requires dose adjustment in clients with renal disease, as it can decrease renal excretion of zand increase its blood levels.
