H1 receptor antagonists

H1 receptor antagonists, also known as H1 antihistamines, are a class of medications that inhibit the action of histamine at the H1 receptor, thereby providing relief from allergic reactions. Histamine is a substance released by the body's immune system during allergic reactions, leading to symptoms such as sneezing, itching, watery eyes, and runny nose. By blocking the H1 receptor, these drugs prevent histamine from exerting its effects, making them effective in treating allergies, such as hay fever, urticaria (hives), and angioedema.

Mechanism of Action

H1 receptor antagonists work by selectively binding to H1 receptors without activating them, effectively blocking histamine from binding and activating these receptors. This blockade prevents the vasodilation, increased vascular permeability, and sensory nerve stimulation that are responsible for the symptoms of an allergic reaction.

Classification

H1 antihistamines can be broadly classified into two categories based on their pharmacological properties and generation:

First-Generation H1 Antihistamines

These drugs, developed earlier, easily cross the blood-brain barrier, often causing sedation as a side effect. Examples include Diphenhydramine, Chlorpheniramine, and Hydroxyzine.

Second-Generation H1 Antihistamines

Developed to minimize unwanted central nervous system effects, second-generation antihistamines are less likely to cross the blood-brain barrier and therefore typically cause less sedation. Examples include Cetirizine, Loratadine, and Fexofenadine.

Indications

H1 receptor antagonists are indicated for the relief of symptoms associated with:

• Allergic rhinitis
• Urticaria
• Conjunctivitis
• Dermatological disorders
• Some may also be used for their sedative properties in the treatment of insomnia or anxiety.

Side Effects

While second-generation H1 antihistamines are less sedating, both generations of drugs can cause side effects. Common side effects include:

• Drowsiness (more common with first-generation antihistamines)
• Dry mouth
• Dizziness
• Nausea
• Blurred vision
• Confusion (particularly in the elderly)

Contraindications

H1 receptor antagonists should be used with caution in individuals with:

• Glaucoma
• Prostatic hypertrophy
• Epilepsy
• Cardiovascular disease
• Elderly patients, due to increased susceptibility to central nervous system effects

See Also

• Histamine antagonist
• Allergic rhinitis
• Urticaria
• Angioedema