Histamine2 Blocker Therapy

Histamine H2-receptor blockers, also called H2-receptor antagonists or H2RAs, are medications that help reduce the production of gastric acid. They can be used to treat gastric and duodenal ulcers; gastroesophageal reflux disease, or GERD; heartburn; and Zollinger-Ellison syndrome, which is a rare condition caused by gastrin-secreting tumors. In addition, histamine H2-receptor blockers can be used to prevent stress-induced ulcers in critically ill patients.
Histamine H2-receptor blockers include cimetidine, which can be administered orally, intravenously, and intramuscularly; famotidine, which is given orally or intravenously; and nizatidine, which is given orally. Now, in the stomach, on gastric parietal cells, histamine, stimulates H2 receptors, to secrete gastric acid to promote digestion.
Histamine H2-receptor blockers obstruct these H2 receptors so histamine can’t bind to them. This results in decreased gastric acid secretion and reduces the acidity of the gastric acid.
Alright, let’s focus on cimetidine. Some of the common side effects of cimetidine are due to its actions on H2 receptors located outside the gastrointestinal tract. So, in the brain, side effects can include headaches, dizziness, and confusion. In the skin, cimetidine may cause pruritus and rash. Cimetidine also binds to androgen receptors, which can lead to sexual side effects like decreased libido, impotence, and gynecomastia. Rarely, cimetidine can cause hematological side effects like agranulocytosis and thrombocytopenia.
In addition, reduced production of gastric acid can favor bacterial growth in the stomach. This is associated with an increased risk of micro-aspiration and lung colonization, leading to pneumonia. Lastly, it’s important to note that prolonged use of histamine H2-receptor blockers for 2 years or more may lead to vitamin B12 malabsorption and subsequent deficiency.