Antihistamines: Classification, Uses, and Advantages

Antihistamines are drugs that block the action of histamine at histamine receptors. They are primarily used to manage allergic reactions and symptoms related to conditions like rhinitis, urticaria, and conjunctivitis. Antihistamines are classified into two categories: first-generation and second-generation.

Classification of Antihistamines

First-Generation Antihistamines

These older drugs are highly sedating and can cross the blood-brain barrier, leading to central nervous system effects such as drowsiness.

• Examples: Diphenhydramine, Chlorpheniramine, Promethazine

Second-Generation Antihistamines

These newer antihistamines are non-sedating, as they do not cross the blood-brain barrier effectively.

• Examples: Cetirizine, Loratadine, Fexofenadine, Desloratadine

Therapeutic Uses

First-Generation Antihistamines

• Allergic conditions (e.g., hay fever, rhinitis, conjunctivitis)
• Urticaria (Hives)
• Motion sickness and nausea (e.g., Dimenhydrinate, Meclizine)
• Insomnia (due to sedative properties)
• Parkinson's disease (reduces tremors)
• Preanesthetic Medication (e.g., Promethazine, Cyproheptadine)
• Appetite Stimulation (Cyproheptadine for anorexia or appetite issues)

Second-Generation Antihistamines

• Allergic rhinitis (hay fever symptoms)
• Chronic urticaria (hives and itching)
• Conjunctivitis (itchy, watery eyes)
• Allergy-related conditions (ideal for daytime use)

Adverse Effects

First-Generation Antihistamines

• Sedation and drowsiness
• Dry mouth, blurred vision, constipation, urinary retention (anticholinergic effects)
• Dizziness and impaired motor skills
• Cognitive impairment (especially in the elderly)

Second-Generation Antihistamines

• Headache
• Dry mouth (less common)
• Dizziness (mild)
• QT prolongation (risk with high doses of some antihistamines)

Advantages of Second-Generation Antihistamines

• Lower sedation compared to first-generation antihistamines
• Fewer anticholinergic effects (e.g., dry mouth, blurred vision)
• Longer duration of action (once-daily dosing)
• Reduced cognitive impairment, especially in the elderly
• Better tolerability for daytime use