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 (Leurs et al., Clin Exp Allergy 2002, PMID 11972592).

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.