According to the British National Formulary (BNF), promethazine hydrochloride is a first-generation sedating antihistamine used in the UK to treat allergy symptoms, nausea and vomiting, short-term insomnia, and for sedation before some procedures. Promethazine is one of the older antihistamines, and it is still widely used because the sedation that makes newer antihistamines undesirable is sometimes exactly what is wanted.
For adults in the UK who are considering or have been prescribed promethazine, this guide explains what promethazine is, what it is used for, who should not take it, and the safety points that matter most, including drowsiness, driving, interactions, side effects, overdose risk, storage, disposal, and when to get advice. It does not give doses. Dosing depends on the product, the reason for taking it, your age, your other medicines and your health, which is why it comes from your label, your pharmacist or your prescriber rather than from a web page.
What Promethazine Is
Promethazine hydrochloride is a first-generation antihistamine of the phenothiazine class. It blocks histamine H1 receptors, and because it crosses into the brain readily, it may also cause drowsiness. Newer antihistamines were designed specifically to avoid that effect.
It also has anti-sickness and anti-nausea properties, which is why its uses extend well beyond allergy.
In the UK it is available in several forms, including tablets and oral solutions, under both branded and generic names. Some products are pharmacy medicines you can buy after speaking to a pharmacist. Others are prescription-only medicine. Which category a product falls into depends on its strength, formulation and licensed indication.
What It Is Used For
Licensed uses in the UK typically include:
Allergy symptoms. It is used primarily to manage allergic conditions, including hay fever, urticaria, itching, allergic skin reactions, allergic rhinitis and allergic conjunctivitis.
Nausea and vomiting. Including motion sickness, and it may also be used to control nausea related to anesthesia or chemotherapy where clinically appropriate. For motion sickness, it is typically taken 30–60 minutes before travel. In some circumstances, it may also be used off-label for nausea and vomiting in pregnancy, though that use is a decision for a clinician.
Short-term sleep difficulty. Some promethazine products can be used to treat short-term insomnia in adults. They are intended for short-term use only and are not suitable for ongoing treatment.
Sedation before procedures, including preoperative and postoperative sedation in clinical settings.
Which of these applies to a given product depends on that product's licence. A promethazine product licensed for allergy is not automatically appropriate for sleep, and vice versa. The packaging and patient information leaflet state what a specific product is licensed for.
Treatment suitability depends on individual circumstances, and your pharmacist or prescriber will advise on whether promethazine is appropriate for you.
Where the Dosing Information Comes From (BNF via NICE)
There is no single dose of promethazine that is right for everyone.
The right dose depends on the product you are using, why you are taking it, your age, your weight in some cases, and your overall health. Different promethazine products are used for different conditions, so the recommended dose can vary.
For a product you have bought or been given, the dose is on the label and in the patient information leaflet inside the pack. Read both.If you are unsure about how much to take, ask your pharmacist or prescriber for advice.
The British National Formulary (BNF), available through NICE, provides prescribing guidance for healthcare professionals. It is designed to support clinical decision-making and should not replace the advice given by your prescriber or pharmacist.
Do not increase your dose if you think the medicine is not working. Taking more promethazine will not necessarily improve your symptoms and may increase the risk of side effects. Speak to your pharmacist or prescriber if your medicine is not helping as expected.
Who Should Not Take It
Promethazine is not suitable for everyone. Situations where it may be inappropriate include:
Children under two years. Promethazine is contraindicated in this age group. Never give it to a child under two, and never give an adult product to a child of any age.
Anyone with a previous allergic reaction to promethazine. Patients receiving promethazine should not take it if they have had allergic reactions or hypersensitivity to promethazine before.
Older adults. Sedating antihistamines carry increased risks in older people, including falls, confusion and urinary retention. Prescribers weigh this carefully.
People with certain medical conditions may need to avoid promethazine or use it with caution. These include some breathing problems, glaucoma, prostate enlargement or difficulty passing urine, epilepsy, liver disease and some heart conditions.
Pregnancy and breastfeeding. Promethazine may be used during pregnancy in some situations following clinical assessment.
People taking certain other medicines. See below.
Promethazine should not be used in comatose patients, and people with relevant health conditions should check with a healthcare professional before use.
This list is not exhaustive. The patient information leaflet gives the full set, and a pharmacist can check your specific situation.
Sedation, Driving and Alcohol
The drowsiness is the point of promethazine in some uses and the main hazard in all of them.
It can last into the next day. People frequently underestimate this. If you take it in the evening, you may still be affected in the morning, and you may not notice the impairment yourself.
Driving. Promethazine can cause severe sedation, not just drowsiness, and this can impair driving, cycling, operating machinery and similar tasks. Do not drive, cycle or operate machinery until you know how it affects you, and not at all if you feel drowsy or slowed. Driving while impaired by a medicine is an offence in Great Britain even when the medicine was legitimately taken.
Alcohol. Do not drink alcohol while taking promethazine. The sedative effects combine, and the result is unpredictable and considerably stronger than either alone.
Other sedating substances. The same applies to anything else causing drowsiness, including some painkillers, sleep medicines, anxiety medicines and certain other antihistamines. This risk is especially important with cns depressants because sedation can be intensified. Extra caution is also needed with other cns depressants, as combined use may increase the risk of respiratory depression.
Interactions
Promethazine interacts with a number of medicine groups. Rather than list specifics, which requires knowing your full medication list to be useful, the categories worth flagging are:
- Other CNS depressants, including opioids, benzodiazepines, some antidepressants and other sedating antihistamines
- Medicines affecting heart rhythm
- Some antiepileptic medicines
- Medicines with anticholinergic effects, where effects add up, additive anticholinergic effects are a particular caution in older adults
- MAOI antidepressants
If you take any regular medicine, ask a pharmacist before taking promethazine. They can check your full list against it in a couple of minutes, especially if you have seizure disorders or other relevant health conditions. This applies to prescribed medicines, medicines bought over the counter, and herbal or supplement products, which interact more often than people expect.
If a clinician has asked you about a specific interaction, for example with an antiepileptic, that question belongs with them or with a pharmacist who can see your records, rather than with a general information page.
Side Effects, Including Blurred Vision and Dry Mouth
Sedation is a common side effect of promethazine. Others reported include drowsiness, dry mouth, blurred vision, dizziness, headache, and constipation.
Seek medical advice promptly if you experience a rash, swelling of the face, lips or throat, difficulty breathing, an irregular heartbeat, unusual movements you cannot control, difficulty passing urine, changes in blood pressure, or if you feel confused or unusually unwell.
The patient information leaflet lists side effects in full, including rarer ones. Suspected side effects can be reported through the MHRA Yellow Card scheme, which anyone can use, not just healthcare professionals.
If Too Much Is Taken
Taking more promethazine than intended can be serious, and this is more likely than people expect with a medicine that makes you drowsy and that people sometimes take again thinking the first dose did not work.
If you think you or someone else has taken too much, contact NHS 111 for advice, or call 999 if the person is very drowsy, difficult to rouse, having difficulty breathing, having a seizure, or has collapsed. Overdose can be especially dangerous in young children. Take the packaging with you if you go to the hospital.
Do not wait to see whether it wears off. Signs of overdose can include extreme drowsiness, agitation, confusion, a fast or irregular heartbeat, unsteadiness and seizures, and these can develop over hours.
Keep promethazine out of the sight and reach of children. Because it is sedating and often kept for occasional use, it tends to sit in cupboards long after it was needed.
Storage and Disposal
Store as stated on the packaging, usually below 25°C, away from direct light, in the original pack. Check the expiry date before use.
Return unwanted or expired medicine to a pharmacy for disposal. Do not put it in household waste or flush it.
Getting Advice
For most promethazine questions, a community pharmacist is the fastest route to a reliable answer, and no appointment is needed. Contact your GP or NHS 111 if symptoms persist, if you are unsure whether promethazine is appropriate for you, or if you are using it for sleep for more than a short period, since ongoing sleep difficulty usually needs its underlying cause addressed rather than a sedating antihistamine.
