Promethazine

Promethazine is a phenothiazine derivative used as an antihistamine, antiemetic, and sedative. It is a first-generation H1 receptor antagonist with anticholinergic properties. It is used to treat allergic symptoms, motion sickness, nausea and vomiting, and to facilitate sedation before surgery. It is also marketed in combination with codeine for cough suppression and in some cases for short-term treatment of insomnia.

Promethazine blocks histamine H1 receptors, dampening allergic responses. It also exhibits anticholinergic effects and mild antagonism at several other receptors, including dopaminergic pathways, which contributes to antiemetic and sedative properties. These actions can cause drowsiness and impair coordination early after dosing.

Available as oral tablets, syrup, rectal suppositories, and injectable forms. Parenteral administration should use intramuscular injection; intravenous use is contraindicated due to risk of severe tissue damage and gangrene upon administration or extravasation. Dosing depends on indication, age, and presentation; always follow label directions.

Contraindicated in hypersensitivity to promethazine, acute porphyria, coma or significant central nervous system depression, and in children under two years of age. Caution in pregnancy and lactation; use when clearly indicated. Common adverse effects include drowsiness, dizziness, dry mouth, blurred vision, and constipation. Anticholinergic effects and extrapyramidal symptoms may occur with prolonged use; alcohol and other CNS depressants can enhance sedation. Rare severe reactions include severe skin reactions and hypotension.