Cetirizine

Zyrtec

Second-generation H1 receptor antagonist (peripheral)

Selective peripheral H1 antagonist. Carboxylic acid structure → poor BBB penetration → minimal sedation compared with first-generation antihistamines like diphenhydramine.

Indications

•Allergic rhinitis, urticaria, allergic conjunctivitis
•Pre-medication in patients with contrast or drug allergy history (paired with steroid)
•Pruritus from non-anaphylactic causes

Dosing

Context	Adult	Pediatric
Adult allergy	10 mg PO daily	—
Pre-contrast prophylaxis	10 mg PO 1 h before contrast (with prednisone 50 mg q13/q7/q1 h)	—

Pharmacokinetics

PO onset 30–60 min. Peak 1 h. Half-life 8 h. Renal excretion 60% unchanged.

Hemodynamic effects

Minimal.

Respiratory effects

None directly.

Side effects

!Sedation (less than diphenhydramine but more than fexofenadine; ~10% of users)
!Dry mouth
!Headache

Contraindications

×Severe renal impairment (reduce dose; CrCl < 30 → 5 mg every other day)

Clinical pearls

★AMBULATORY ANESTHESIA: when an H1 blocker is needed pre-op, cetirizine is preferable to diphenhydramine — far less postoperative sedation and Beers-flagging.
★CONTRAST PROPHYLAXIS: 'Greenberger protocol' — prednisone 50 mg PO at 13 h, 7 h, 1 h before contrast + cetirizine or diphenhydramine 1 h before. Reduces moderate-severe reactions by ~80%.
★RENAL FAILURE: dose reduction matters more than for diphenhydramine because cetirizine is renally cleared.
★PRURITUS NEURAXIAL OPIOID: cetirizine is less effective than naloxone or nalbuphine because the itch is opioid-receptor-mediated, not histaminergic.

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Education only — confirm against current package inserts and institutional protocols. Doses assume normal organ function unless otherwise noted.