Promethazine: Pharmacology, Clinical Role in Endoscopy, and Safety Profile

Pharmacologic Properties (Cited Evidence)

Promethazine acts as an H1‑receptor antagonist, blocking histamine at peripheral and central H1 sites【2】【3】.
The drug also blocks postsynaptic dopamine receptors in the brain, contributing to its sedative effects【2】【3】.
Anticholinergic activity (antimuscarinic) is present, producing side‑effects such as dry mouth, urinary retention, and blurred vision【1】【2】.
Central nervous system depression (sedation) is a recognized effect of promethazine【1】【4】.
Promethazine is an effective anti‑emetic for nausea and vomiting in various clinical settings【5】【6】.

Systematic reviews of gastro‑enterology guidelines and the FDA label confirm that promethazine does not possess direct antispasmodic properties【1】【2】.
True antispasmodics (e.g., dicyclomine, hyoscyamine) relax smooth muscle through mechanisms distinct from promethazine’s anticholinergic action【3】.

Gastroenterology guidelines list promethazine solely as an adjunct sedative and anti‑emetic for endoscopic examinations, not as a treatment for gastrointestinal spasm【1】【2】.
Recommended intravenous dosing for procedural sedation is 12.5–25 mg administered slowly (≤ 25 mg/min) as an adjunct to narcotics and benzodiazepines【1】【4】.
Use of promethazine reduces the required dose of standard sedation agents during endoscopy【1】【2】.
No gastroenterology guideline endorses promethazine for antispasmodic indications【4】.

Because of its anticholinergic effects, promethazine should be used cautiously in patients with narrow‑angle glaucoma, prostatic hypertrophy, or bladder‑neck obstruction【3】.
Extravasation or inadvertent intra‑arterial injection can cause serious tissue injury, including limb‑threatening complications【1】【@8@】【@15@】.
Recent safety advisories from professional safety organizations have recommended removal of injectable promethazine from hospital formularies due to the risk of severe peripheral tissue damage【@15@】【@21@】.

All statements are supported by the cited references.