Difference Between H1 and H2 Blockers

The key difference between H1 and H2 blockers is that H₁ blockers refer to compounds that inhibit the activity of the H₁ histamine receptors that occur throughout the vascular endothelial cells in the heart and central nervous system, while H₂ blockers refer to compounds that inhibit the activity of the H₂ histamine receptors that mainly occur in the parietal cells of the gastric mucosa.

Antihistamines are drugs used by doctors to treat hay fever and allergies. Generally, people take antihistamines without prescriptions as they have few side effects. They relieve symptoms such as nasal congestion, sneezing, or hives caused by pollen, dust mites, or animal allergy. But they are usually for short-term treatment. There are several types of antihistamines. H1 and H2 blockers are two main types of antihistamines used to treat allergic reactions.

1. Overview and Key Difference
2. What are H1 Blockers
3. What are H2 Blockers
4. Similarities – H1 and H2 Blockers
5. H1 vs H2 Blockers in Tabular Form
6. Summary – H1 vs H2 Blockers

What are H1 Blockers?

H₁ blockers refer to compounds that inhibit the activity of the H₁ histamine receptors. They occur throughout the vascular endothelial cells in the heart and central nervous system. They are also called H1 antagonists or H1 antihistamines. They help to relieve the symptoms of allergic reactions. H₁ histamine receptors exhibit constitutive activity. Therefore, H₁ blockers can be either neutral receptor antagonists or inverse agonists. Neutral receptor antagonists work by binding to the H1 receptor and blocking the activation of the receptor by histamine. On the other hand, inverse agonists work by binding to the H1 receptor and blocking the binding of histamine, and reducing the constitutive activity of the H1 receptor.

Figure 01: H1 Blockers – Cetirizine

In the clinical setup, H1 blockers are used to treat allergic reactions and mast cell disorders. Sedation is a commonly found side effect of H1 blockers. Therefore, they are (diphenhydramine and doxylamine) normally used to treat insomnia. Moreover, H1 blockers can also be used to reduce inflammatory reactions. Some of the examples for H1 blockers are acrivastine, buclizine, cetirizine, desloratadine, hydroxyzine, levocetirizine, maprotiline, promethazine, phenyltoloxamine, orphenadrine, tripelennamine, etc.

What are H2 Blockers?

H2 blockers refer to compounds that inhibit the activity of H2 histamine receptors. They mainly occur in the parietal cells of the gastric mucosa. They are also called H2 antihistamine or H1 antagonists (H2RAs). They normally exist as inverse agonists and neutral antagonists. These H2 antihistamines act upon H2 histamine receptors, mainly in the parietal cells of the gastric mucosa. Parietal cells of the gastric mucosa are a part of the endogenous signalling pathway for gastric acid secretion. Usually, histamine acts upon H2 receptors to induce acid secretion. Thus, H2 blockers inhibit H2 signalling and reduce the secretion of gastric acid.

Figure 02: H2 Blockers – Cimetidine

H2 blockers are the first-line treatment for gastrointestinal conditions, including peptic ulcers and gastroesophageal reflux disease. Further, they are also used for the treatment of dyspepsia. The common examples of H2 blockers are cimetidine, ranitidine, famotidine, nizatidine, roxatidine, lafutidine, lavoltidine, and niperotidine, etc.

What are the Similarities Between H1 and H2 Blockers?

Both are types of antihistamines.
They block the histamines receptors.
They are used for the treatment of human diseases.
Both exist as neutral receptor antagonists or inverse agonists.

What is the Difference Between H1 and H2 Blockers?

H₁ blockers refer to compounds that inhibit the activity of the H₁ histamine receptors that occur throughout the vascular endothelial cells in the heart and central nervous system. On the other hand, H₂ blockers refer to compounds that inhibit the activity of the H₂ histamine receptors that mainly occur in the parietal cells of the gastric mucosa. Thus, this is the key difference between H1 and H2 blockers. Furthermore, H₁ blockers were discovered in 1933, while H2 blockers were found much later in 1964.

The following infographic lists the differences between H1 and H2 blockers in tabular form.

Summary – H1 vs H2 Blockers

Currently, most people use antihistamines to treat allergies. The first generation of antihistamines was available from the 1930s. H1 and H2 blockers are two antihistamines. H1 blockers affect the nose, while H2 blockers affect the stomach. Furthermore, H₁ blockers refer to compounds that inhibit the activity of the H₁ histamine receptors, while H₂ blockers refer to compounds that inhibit the activity of the H₂ histamine receptors. Thus, this is the summary of the difference between H1 and H2 blockers.