Antihistamines
Famotidine
Pepcid
H2 receptor antagonist
Reversible competitive antagonist at parietal cell H2 receptors → reduces basal + stimulated gastric acid secretion. Raises gastric pH within 30 min.
Indications
- •Aspiration prophylaxis (with metoclopramide + non-particulate antacid — full-stomach, OB, obese, GERD)
- •Anaphylaxis adjunct (paired with H1 blocker after epinephrine)
- •GERD / PUD treatment
- •Stress-ulcer prophylaxis in ICU (largely replaced by PPIs)
Dosing
| Context | Adult | Pediatric |
|---|---|---|
| Aspiration prophylaxis preop | 20 mg IV/PO 30–60 min before induction | — |
| Anaphylaxis | 20 mg IV after epinephrine | — |
Pharmacokinetics
Onset 30 min IV. Peak 1 h. Duration 10–12 h. Renal elimination — reduce dose CrCl < 50.
Hemodynamic effects
Minimal at clinical doses.
Respiratory effects
None.
Side effects
- !Headache
- !Confusion (rare; elderly + renal failure)
- !Thrombocytopenia (rare)
Contraindications
- ×Hypersensitivity to H2 blockers
Clinical pearls
- ★FULL-STOMACH RAPID SEQUENCE: 20 mg IV is part of the standard prep along with metoclopramide 10 mg IV (gastric emptying) and 30 mL 0.3 M sodium citrate (immediate non-particulate buffer).
- ★TIMING MATTERS: PO 60 min preop. IV 30 min preop. < 30 min before induction = inadequate effect.
- ★RENAL DOSE: CrCl < 50, halve the dose; CrCl < 10, quarter.
- ★VS. PPIs: PPIs are more potent acid suppressors but slower onset (2–4 h). Famotidine wins for same-day surgery.
Other drugs in Antihistamines
- Diphenhydramine
Competitive H1 antagonist with significant anticholinergic + sedating activity from CNS penetration. Crosses BBB freely.
- Cetirizine
Selective peripheral H1 antagonist. Carboxylic acid structure → poor BBB penetration → minimal sedation compared with first-generation antihistamines like diphenhydramine.
Browse all classes: /reference/drugs
Suggested reading
- •Stoelting & Hines, Pharmacology & Physiology in Anesthetic Practice, 6e
- •Miller's Anesthesia, 9e
- •FDA package insert
Education only — confirm against current package inserts and institutional protocols. Doses assume normal organ function unless otherwise noted.
