Most people tolerate promethazine well, but drowsiness or sedation is the most common side effect. Dry mouth, dizziness, constipation and blurred vision can also occur, while restless legs syndrome and other movement-related effects have also been reported.
This guide is for adults in the UK who are using or considering promethazine, parents of children who may be prescribed it, and anyone who wants a clear view of side effects, safety and interaction risks before taking it.
It covers effects ranging from common and manageable symptoms to less common and potentially serious reactions, including severe allergic reactions, blood disorders, liver problems and breathing difficulties. It also looks at questions about restless legs, weight gain, age-related safety in children, medicines and alcohol that can make side effects worse, and what you can do if symptoms appear. Knowing which effects can be expected, which need medical advice, and when to seek urgent help can make treatment safer.
Key things to know
- Drowsiness or sedation is the only adverse effect classified as "very common" in the UK Phenergan product information.
- Restless legs syndrome and other extrapyramidal effects are recognised adverse effects, although their frequency is not known.
- Weight gain is not listed as an adverse effect in the current UK Phenergan product information.
- Phenergan 25mg tablets and Phenergan Elixir are contraindicated in children under 6 years, age restrictions vary between promethazine products.
The Most Common Effect: Sedation
Sedation (drowsiness) is the only side effect officially classified as "very common" in the current UK product information for promethazine. Other recognised effects, including dry mouth, dizziness, constipation and blurred vision, are listed with a frequency of "not known", meaning their frequency cannot be estimated from the available data.
You may see websites quoting specific percentages for promethazine side effects, but unless those figures can be traced to reliable clinical evidence or official product information, they should be treated cautiously. This article does not use unverified estimates.
Commonly and Milder Promethazine Side Effects
Some of the more manageable side effects associated with promethazine include:
- Drowsiness or sedation. Very common.
- Dry mouth.
- Dizziness.
- Constipation.
- Blurred vision.
The UK product information lists drowsiness or sedation as very common, while the other effects above have a frequency of "not known", meaning their frequency cannot be estimated from the available data. If promethazine makes you drowsy, dizzy or causes blurred vision, avoid driving or operating machinery until you know how it affects you.
Restless Legs: What is Actually Going On
Promethazine can cause restless legs syndrome and other extrapyramidal effects, although the UK product information lists their frequency as "not known", meaning the frequency cannot be estimated from the available data.
Restless legs syndrome is not the same as akathisia. Restless legs syndrome typically causes an urge to move the legs accompanied by uncomfortable sensations, often becoming more noticeable during periods of rest and in the evening or at night. Akathisia is a different type of movement-related restlessness characterised by an uncomfortable need to keep moving.
If you develop new or persistent restlessness, an unusual urge to move, muscle spasms or other involuntary movements after starting promethazine, speak to a pharmacist or prescriber. These effects are recognised in the UK product information and should not simply be ignored.
The reassuring context is that this appears to occur less often with promethazine than with some other phenothiazine medicines, it is specifically noted as being among the lowest-incidence options in this drug class for this type of effect. It can also lower the seizure threshold and, in predisposed people, cause seizures, so if you have seizure disorders this is something to raise with your prescriber. It is also worth knowing that clinical restless legs syndrome, a specific condition involving leg sensations mainly in the evening, relieved by movement, is technically a different thing from akathisia, even though they can feel similar. If you experience genuine restlessness or an inability to stay still after taking promethazine, this is a real, recognised effect worth mentioning to your prescriber or pharmacist, particularly if it is persistent or distressing, rather than ignoring it or continuing treatment without seeking advice.
Promethazine can occasionally have the opposite of its expected sedating effect. Children may experience paradoxical hyperexcitability, restlessness, nightmares or behavioural changes. This is one reason age-appropriate use and professional advice are particularly important when promethazine is given to children.
Weight Gain: An Honest Answer
Weight gain is not listed as an adverse effect in the current UK Phenergan product information for promethazine. Some sedating antihistamines are anecdotally linked to appetite changes, and reduced activity from sedation could plausibly play a role for some people, but we are not able to confirm a direct, established effect specific to promethazine. If you notice unexplained weight changes while taking promethazine, particularly with longer-term use, that's worth mentioning to your prescriber rather than assuming it either is or isn't related.
This is a good example of where being honest about the limits of what we can confirm matters more than giving a confident-sounding but unverified answer either way. A genuine "we're not sure, but here's what we do know" is more useful to you than false certainty.
Serious Promethazine Side Effects
These appear to be uncommon, but it is worth knowing what to look for so you can tell the difference between something to manage at home and something needing urgent attention, as respiratory depression is a rare but serious side effect:
- Signs of a severe allergic reaction: swelling of the face or throat, difficulty breathing, severe rash.
- Slow or shallow breathing can be a separate warning sign and may need urgent medical attention.
- Signs of a blood disorder: unusual bruising or bleeding, sore throat with fever, or other signs of bone marrow suppression affecting blood cells, including low white blood cells, which can increase the risk of serious infections.
- Signs of a liver effect: yellowing of the skin or eyes.
- Confusion, hallucinations, or significant changes in mental state.
- Neuroleptic malignant syndrome has also been reported with promethazine. Symptoms such as high fever, severe muscle rigidity and altered consciousness require urgent medical attention.
These symptoms should not be assumed to be normal effects of promethazine. Seek urgent medical help for difficulty breathing, severe allergic symptoms, significant confusion, jaundice, unusual bleeding, a fast or irregular heartbeat, or severe muscle stiffness or abnormal movements.
A Correction on Children
Promethazine products have different age restrictions depending on the formulation and indication. Always check the specific product information and follow advice from a healthcare professional. For example, the UK product information for Phenergan 25mg tablets and Phenergan Elixir states that these products must not be used in children under 6 years of age. NHS guidance notes that some children aged 2 to 5 may be prescribed certain promethazine liquids, so parents should check the specific product rather than applying one age limit to every promethazine medicine.
Promethazine should not be used in children under 6 years with the Phenergan formulations covered above because of the potential risk for serious respiratory, psychiatric and central nervous system effects. Children can also be more susceptible to paradoxical excitation, restlessness and behavioural effects.
Older adults can also be particularly susceptible to anticholinergic effects and confusion.
For the fuller picture on contraindications and who should avoid promethazine entirely, see our promethazine safety guide. Older adults face particular sensitivity to several of these effects, especially confusion and falls, which is covered in more detail there.
Drug Interactions That Worsen Side Effects
- Other sedating medicines (painkillers, benzodiazepines, sleep aids): as other CNS depressants and CNS depressants, these can amplify sedation, cause severe drowsiness, and increase breathing suppression risks when combined.
- Alcohol: avoid drinking alcohol while taking promethazine, as it significantly increases sedation and risk.
- MAOIs: Promethazine should be avoided in people taking a monoamine oxidase inhibitor (MAOI) or who have taken one within the previous 14 days.
- Some antidepressants, including tricyclic antidepressants, can increase anticholinergic side effects like dry mouth and constipation. Ask a pharmacist or prescriber to check your medicines if you take promethazine alongside other medicines with anticholinergic effects.
- Muscle relaxants and other medicines: can also intensify sedation.
Always tell your prescriber or pharmacist about other drugs and other medicines you are taking, including over-the-counter products and supplements, before starting promethazine.
Managing Side Effects Sensibly
If promethazine makes you drowsy, dizzy or affects your concentration, avoid driving, cycling or using machinery until you know how the medicine affects you. Drowsiness can last for several hours after a dose.
For dry mouth or constipation, staying hydrated and asking a pharmacist about suitable self-care measures may help. If a side effect is persistent, severe or troubling, speak to a pharmacist or prescriber rather than changing the dose yourself.
Any dose changes should be guided by your prescriber based on your response, not adjusted independently in an attempt to find your own "right" amount. If you have a missed dose, take it only if appropriate, otherwise wait for the next dose, and never use double or extra doses.
Keeping a simple note of what you notice, when it started, how severe it feels, and whether it is improving, staying the same, or worsening, makes it much easier to have a useful conversation with a pharmacist or prescriber than trying to describe a vague general impression after the fact.
If you have taken promethazine regularly for a long time, speak to your doctor before stopping suddenly, as withdrawal symptoms can occur after prolonged use.
Have questions about a specific side effect? Start your consultation and our clinical team can help.
Available Treatments
| Treatment | Price |
|---|---|
| Phenergan 25mg (Promethazine Hydrochloride) - 56 Tablets | £14.99 |
| Phenergan Elixir 5mg/5ml Oral Solution - 100ml | £14.49 |
This guide discusses oral tablet and oral solution formulations. Promethazine is also available as an injection, which has separate prescribing considerations and safety information. Serious tissue injury has been reported with promethazine injection, including at the injection site, and subcutaneous or intra-arterial administration is contraindicated.
Prices update live and may change.
Medical Disclaimer
This page provides general information about promethazine side effects. It is not a substitute for personalised medical advice, diagnosis, or treatment. If you experience signs of a serious reaction, including swelling of the face or throat, difficulty breathing, high fever with muscle rigidity, or significant confusion, seek medical attention promptly. If you have questions about a specific side effect, pregnancy, breastfeeding, or interactions with other medicines, speak to your prescriber, pharmacist, GP, or healthcare provider.
