Dermatology/H1-antihistamine

Antihistamines (H1)

also: cetirizine · loratadine · fexofenadine · chlorphenamine · Piriton

Overview

H1-antagonists for urticaria, allergic rhinitis and pruritus. Non-sedating agents are first-line; sedating agents are reserved for nocturnal itch (cross-references ENT/allergy).

Mechanism

Inverse agonists at the H1 histamine receptor → block histamine-mediated wheal, flare, itch and vasodilation. Sedating (1st-generation) agents cross the blood–brain barrier and have anticholinergic effects; non-sedating (2nd-generation) agents are peripherally selective.

Indications

Urticaria & angioedema
Allergic rhinitis / conjunctivitis
Pruritus
Allergic reactions (adjunct, after adrenaline in anaphylaxis)

The agents

Cetirizine / loratadine / fexofenadineNON-sedating (2nd gen)

First-line; up-dose for chronic urticaria.

Chlorphenamine (Piriton)SEDATING (1st gen)

short-acting

Anaphylaxis adjunct; nocturnal itch.

Hydroxyzine / promethazineSEDATING

Marked sedation; itch + nausea.

Adverse effects

Sedation (1st-generation)classic

Impairs driving; avoid in the elderly.

Anticholinergic effects (dry mouth, urinary retention, confusion)common

1st-generation.

Non-sedating agents: minimal CNS effectscommon

Cautions & contraindications

Sedating antihistamines in the frail elderlyelderly

Falls, confusion, anticholinergic burden.

Caution: prostatic enlargement, glaucoma (1st-gen anticholinergic)all

Interactions

Additive sedation/anticholinergic load with alcohol, opioids, TCAs

Monitoring & kinetics

Symptom control; sedation

Oral. Non-sedating first-line; chronic urticaria may need up to 4× standard dose (specialist).

Source: NICE CKS — Urticaria / allergic rhinitis · BNF — Antihistamines