A histamine blocker is a type of medication that modifies the activity of histamine, a specialized amine that functions as a neurotransmitter and an inflammatory mediator. Histamine blockers are often referred to as antihistamines. However, this term is misleading. For one thing, these medications do not block the synthesis of histamine from the amino acid histidine as the term implies, nor do they prevent its release from mast cells. Instead, a histamine blocker is designed to prevent targeted receptor sites from accepting the chemical’s attachment, as well as the instructions it’s programmed to deliver.
There are two types of histamine receptors that receive histamine, but they respond differently. H1 receptors are involved in inflammatory reactions and produce symptoms commonly seen in allergic reactions, such as swelling, sneezing, and increased nasal secretions. Therefore, an H1 histamine blocker, also known as an H1 antagonist, is given to treat allergies. H2 receptors, on the other hand, are involved in regulating the secretion of gastric acid from parietal cells located in the lining of the stomach. So, an H2 histamine blocker, or H2 antagonist, is used to prevent excess production of stomach acid and is intended for the treatment of peptic ulcers and gastroesophageal reflux disease (GERD).
Examples of common blockers that act upon H1 receptor sites include diphenhydramine (Benadryl®), loratadine (Claritin®), and chlorpheniramine (Chlor-Trimeton®). Some of these medications, particularly first generation drugs like diphenhydramine and chlorpheniramine, also impact H1 receptors in the brain, which triggers a sedative effect. However, some of the newer medications, like loratadine, produce far less drowsiness because these drugs cannot pass though the blood-brain barrier as well as their predecessors. The most commonly used H2 blockers are cimetadine (Tagamet®) and ranitidine (Zantac®). These medications are available without a prescription.
While these medications are very effective and considered safe, they are not without risk and side effects. For example, some of these drugs produce drowsiness and may interfere with the ability to drive or operate machinery, an effect enhanced with alcohol consumption. In contrast, non-sedating blockers may produce insomnia in some people. Other common side effects include dry mouth, dizziness, hyperactivity, irregular heartbeat, flushing of the skin, and stomach cramps. In addition, the prescription-based drug cimetidine is contraindicated with warfarin (Coumadin®) and other blood thinners due to an increased risk of bleeding, increases serum levels of many other medications, such as calcium channel blockers and cholesterol-lowering drugs, and is known to pass through breast milk.