A sedating (older) antihistamine
Promethazine
A sedating older antihistamine used for allergies, short-term sleep problems, and nausea or travel sickness.
What is Promethazine?
Promethazine is one of the older, first-generation antihistamines. Unlike modern non-drowsy antihistamines it crosses into the brain and causes drowsiness, which is why it is used for allergy symptoms, itching, feeling sick, and short-term sleep problems.
Education and reference only. This is a plain-language guide to Promethazine — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Promethazine is one of the older, "first-generation" antihistamines. Unlike the modern non-drowsy antihistamines, it crosses into the brain and causes drowsiness, which is both its main side effect and, in some cases, the point of using it. It is used for allergic conditions such as hay fever and itchy rashes, for short-term relief of sleep problems in adults, and to prevent or treat nausea and travel sickness. In the UK it is widely known as Phenergan, and the sleep product Sominex contains it. Several forms are available from pharmacies without a prescription.
How it works
Promethazine blocks histamine, a chemical the body releases during allergic reactions that causes itching, sneezing, a runny nose and swelling. Because it also gets into the brain and blocks histamine there — as well as having some other calming actions — it causes drowsiness and helps with sleep and the queasiness of motion sickness. This brain effect is what separates it from the modern antihistamines that are designed to stay out of the brain and so cause little or no drowsiness.
Company & origin
Originated / developed by: Rhone-Poulenc (now Sanofi).
Promethazine, a phenothiazine antihistamine, was synthesized by Paul Charpentier at Rhone-Poulenc in France in the mid-1940s and introduced into clinical use around 1947.
What it treats
Conditions Promethazine is used for
Practical use
How to take Promethazine
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Expect it to cause drowsiness, so avoid driving or operating machinery if affected, especially the day after taking it for sleep.
- Avoid alcohol, which adds strongly to the sedation.
- When used for travel sickness, take it before the journey so it is working in time.
- Use it only for short-term sleep problems rather than long-term, and follow pharmacy or prescriber advice.
- Check with a pharmacist before combining it with other sedating medicines or cold and flu remedies.
- It is not suitable for very young children — always check the age limits before use.
Weighing it up
Advantages & disadvantages of Promethazine
Advantages
- Helps with allergy symptoms, itching, nausea and short-term sleeplessness.
- Useful for travel sickness when taken in advance.
- Long-established medicine that is widely available.
- Its sedating effect can be helpful when sleep is disturbed.
Disadvantages
- Causes marked drowsiness that can carry over into the next day.
- Can cause dry mouth, blurred vision, constipation and dizziness.
- Adds dangerously to the effects of alcohol and other sedatives.
- Not suitable for young children and best avoided long term.
Practical use
Good to know
The drowsiness can carry over into the next morning ("hangover" effect), so take care with driving and tasks needing concentration, and be cautious about combining it with alcohol or other sedating medicines. For sleep it is meant only for short-term use, not as an ongoing solution. For travel sickness it works best taken before the journey, before symptoms start. It is not suitable for young children, and it should be avoided in very young infants entirely. Its anticholinergic effects (dry mouth, blurred vision, constipation) make it less suitable for older people.
Who should not take it / use with caution
- Young children — it is not given to very young infants, and is used with care and according to age advice in older children.
- People with significant breathing problems, an enlarged prostate or urinary retention, certain types of glaucoma, or severe liver disease.
- Used with caution in older people (because of drowsiness, confusion and falls) and alongside alcohol or other sedating medicines.
Monitoring
- No routine blood tests
- Daytime drowsiness and its effect on driving and concentration
- In older people, any confusion, falls or urinary problems
Side effects
- Drowsiness, which can persist into the next day, plus dry mouth, blurred vision and constipation.
- Dizziness, a "hungover" feeling, or — particularly in children and older people — sometimes restlessness or agitation instead of calm.
- Less commonly, difficulty passing urine, a faster heartbeat; rarely, more serious effects needing review.
Key interactions
- Adds strongly to the drowsiness of alcohol, sleeping tablets, strong painkillers and other sedating medicines.
- Adds to the anticholinergic (dry mouth, constipation, blurred vision) effects of some other medicines.
- Care alongside certain medicines that affect heart rhythm, and it is not used close to MAOI antidepressants.
Available as: Tablets and a liquid (oral solution); some forms are available without a prescription. Different products are marketed for allergy, sleep and travel sickness.
Answers
Promethazine: frequently asked questions
Will promethazine make me drowsy the next day?
It can — promethazine is long-acting and the drowsiness sometimes carries over into the next morning, sometimes called a "hangover" effect. Take care with driving and anything needing concentration until you know how it affects you, and avoid alcohol, which makes the drowsiness worse.
Can I give promethazine to my child?
Only with care and according to the age advice on the product — it is not suitable for very young infants and can occasionally cause agitation or breathing problems in small children. Always check the label and ask a pharmacist; never use it to sedate a child for convenience.
Can I use promethazine every night to help me sleep?
It is intended only for short-term help with sleep, not as an ongoing nightly remedy. Using a sedating antihistamine long-term is not recommended, and persistent sleep problems are better assessed and addressed at the cause. Speak to your pharmacist or GP if you need it for more than a short period.
How is promethazine different from antihistamines like cetirizine or loratadine?
Promethazine is an older, sedating antihistamine that gets into the brain and causes drowsiness, whereas cetirizine and loratadine are modern "non-drowsy" antihistamines designed to stay out of the brain. For everyday hay fever the non-drowsy ones are usually preferred; promethazine is chosen when its sedating or anti-sickness effect is wanted.
What is the difference between promethazine, Phenergan and Sominex?
They contain the same active ingredient — promethazine is the generic name, Phenergan is a well-known brand, and Sominex is a sleep product containing it. The main difference is which use each product is marketed for (allergy, sleep or travel sickness).
The wider class
About Antihistamines
Promethazine belongs to the antihistamines class. For how the class as a whole works, its shared safety principles and monitoring, see the full guide.
Browse by body system
Authoritative sources
- BNF: Promethazine hydrochloride.
- electronic Medicines Compendium (SmPC): Phenergan.
- NICE CKS: Promethazine.
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