Promethazine: uses, drowsiness and safety notes

The NHS describes promethazine as an antihistamine that relieves allergy symptoms, and as a drowsy one. It also blocks dopamine, which helps prevent nausea. It is used for hay fever and hives, for sickness from motion sickness or vertigo, and occasionally on prescription for itching in pregnancy caused by intrahepatic cholestasis.

Tablets come as 10 mg, 20 mg or 25 mg, and the liquid is 5 mg per 5 ml. Combination products with paracetamol or dextromethorphan are sold for cough, cold and pain; check the ingredients before taking anything else.

Most adults can take it. For children, the NHS says liquid can be prescribed from 2 to 5 years and bought without prescription from 6, and tablets can be taken from 5 for allergies or sickness. Do not give cough and cold medicines containing promethazine to children aged 5 or under.

The NHS gives examples for adults and children over 10. For hay fever or hives it is 10 mg twice daily up to 25 mg once or twice daily. To prevent motion sickness, take 25 mg the night before travel; to treat it, 25 mg immediately, then 25 mg the same evening. For vertigo, 25 mg daily up to four times daily. Your pharmacist or prescriber will tell you what applies to you.

Only take it for a short time unless a doctor says otherwise. If you miss a dose for motion sickness, take it when you remember; for other uses skip it. Never double up.

Drowsiness usually wears off within 12 hours of a dose.

• Common: daytime drowsiness, nightmares, dizziness or unsteadiness, difficulty concentrating, and headaches.

• Avoid driving or cycling if drowsy or dizzy. Avoid strong sunlight, because it can cause sun sensitivity.

• Call NHS 111 for confusion, blurred vision, difficulty urinating, yellowing of the eyes or skin, unusual bruising or bleeding, muscle stiffness or abnormal movements, a pounding heartbeat, or aggression or hallucinations in a child.

• Call 999 for anaphylaxis.

Tell the pharmacist or doctor about primary angle closure glaucoma, difficulty emptying the bladder, epilepsy, asthma or breathing problems, an enlarged prostate, heart problems, and liver or kidney disease. It can affect allergy tests and home pregnancy tests, so mention it beforehand.

Do not drink alcohol while taking it. The NHS lists antidepressants including MAOIs, amiodarone, anticonvulsants, erythromycin, codeine, morphine and oxycodone, co-codamol, and any medicine causing drowsiness, dry mouth or urination difficulty. Do not combine two drowsy antihistamines.

The NHS says promethazine is safe in pregnancy with no evidence of harm, although it may cause drowsiness, and a non-drowsy option such as loratadine may be suggested for hay fever. It is likely to pass into breast milk only in low levels, best for occasional or short-term use, and avoid bed-sharing with your baby. Suspected side effects can be reported to the MHRA Yellow Card scheme.