Chlorphenamine (Piriton): uses, drowsiness and cautions

Chlorphenamine relieves allergy symptoms. The NHS lists hay fever, red itchy eyes, eczema, hives caused by food allergy or chickenpox, and insect bites and stings. It comes as 4mg tablets or as a liquid with 2mg in 5ml. Some cough and cold liquids combine it with ephedrine or pseudoephedrine, and those have their own labels.

Unlike cetirizine, fexofenadine and loratadine, chlorphenamine tends to make you sleepy. Most people prefer a non-drowsy option for daytime, but a sedating one may help when itching stops you sleeping, and doctors sometimes pair a daytime non-drowsy antihistamine with chlorphenamine at night for a few days for a very itchy rash.

The NHS says do not use it purely as a sleeping aid without talking to a doctor, and do not combine it with another drowsy antihistamine such as diphenhydramine (Nytol) or promethazine (Phenergan), as the result will be heavy tiredness. It is not known to be addictive.

For adults and children aged 12 and over, the NHS gives the usual dose as 1 tablet every 4 to 6 hours, with no more than 6 tablets in 24 hours. Children under 12 have lower and less frequent doses, according to the pack or the prescriber. Babies under 1 year should only take chlorphenamine prescribed by a doctor, and cough or cold medicines containing it must not be given to children under 6.

Frail older adults usually take 1 tablet every 6 to 8 hours, no more than 3 tablets in 24 hours, and should speak to a doctor or pharmacist first because side effects are more likely. Take it with or without food, swallowing tablets with water, milk or juice. The 4mg tablet can be halved for lower doses. Use the syringe or spoon with liquid, not a kitchen teaspoon.

Only take it when you need to. For hay fever you may take it regularly through the season, but do not go beyond 2 weeks without a doctor, and see a doctor if symptoms do not improve within a few days. If you miss a dose, take it when you remember unless the next is nearly due.

Common effects include nausea, sleepiness and dizziness, and also difficulty concentrating, dry mouth, headaches and blurred vision. Do not drive or ride a bike if you are sleepy, dizzy, clumsy, have blurred vision or cannot concentrate, which is likelier when you first start or when you add another medicine. Swelling of the lips, mouth, throat or tongue needs 999. Suspected side effects can be reported through the Yellow Card scheme.

The NHS warns that taking too much chlorphenamine can be dangerous, and advises calling 111 even if you have no symptoms. Call 999 or go to A&E if, after an excess dose, there is a very fast, irregular or pounding heartbeat, breathing problems, a seizure, loss of consciousness, uncontrolled movements, especially in the hands or feet, confusion, agitation or difficulty speaking, or hallucinations. Take the packaging to hospital and do not drive yourself.

Tell a doctor or pharmacist if you have ever reacted to chlorphenamine or other medicines, have primary angle closure glaucoma, have taken a monoamine oxidase inhibitor antidepressant in the last 14 days, struggle to pass urine or empty your bladder, have an enlarged prostate, epilepsy, asthma or other breathing problems, heart disease, very high blood pressure, liver or kidney problems, or cannot have alcohol (some liquids contain a little). Frail older adults may become confused. If you are due an allergy test, it may affect the result and may need stopping a few days before.

The NHS names monoamine oxidase inhibitors such as phenelzine, phenytoin, and any medicines for anxiety or sleep, plus anything that causes drowsiness, dry mouth or difficulty passing urine. Paracetamol and ibuprofen are safe alongside it, but painkillers that contain codeine, such as co-codamol, or other prescription painkillers make side effects more likely.

Chlorphenamine can be taken in pregnancy and there is no good evidence of harm, although drowsiness can occur and a non-drowsy option such as loratadine may be recommended. If your baby is healthy you can take it while breastfeeding, but a non-drowsy antihistamine might be better, and it is best only occasional. Do not share a bed with your baby while taking it. Contact a professional or 111 if your baby feeds poorly or seems unusually sleepy or irritable.