H1 Antihistamines

H1 antihistamines compete with Histamine for binding at the H1 Receptor. They are inverse agonists — they don’t just block histamine from binding, they actively shift the receptor toward its inactive conformation.

First Generation

Diphenhydramine (Benadryl), hydroxyzine, chlorpheniramine

These cross the blood-brain barrier readily. This causes sedation (blocking wakefulness signaling) and can impair cognition. They also have anticholinergic effects (dry mouth, urinary retention, constipation).

Clinically useful: sedation at bedtime can be channeled productively. Hydroxyzine also has mild anxiolytic properties, which may address the stress-mast cell component.

Short-acting (4-6 hours for diphenhydramine), requiring multiple daily doses.

Second Generation

Cetirizine (Zyrtec), loratadine (Claritin), fexofenadine (Allegra)

Designed with reduced blood-brain barrier penetration → less sedation. Cetirizine is mildly sedating in some individuals; fexofenadine is the least sedating.

Longer-acting (12-24 hours), allowing once or twice daily dosing. Generally preferred for daytime use.

What They Don’t Do

H1 antihistamines only block one of four histamine receptors. They don’t touch H2-mediated symptoms (gastric acid, some cardiac effects), don’t affect Prostaglandins, Leukotrienes, Cytokines, or any non-histamine mediator. They manage symptoms but don’t prevent Degranulation — the mast cell is still firing, and all mediators are still being released. Only the H1-mediated downstream effects are blocked.