Some common problems with Phenergan tablets include feeling drowsy the next day, finding that the medicine does not seem to help as much as expected, or making mistakes with how the tablets are taken. Some people may also find the effect feels too strong, particularly because promethazine is a sedating antihistamine.
This guide is for adults in the UK who use or are considering Phenergan tablets for allergies, motion sickness or short term insomnia and want practical information when things do not go as expected. It looks at common problems and explains what the UK product information and NHS guidance say about Phenergan tablets, including tablet strengths, how to take them, factors that may affect sleep related use, possible side effects and interactions, common mistakes, UK availability, and when it is sensible to speak to a pharmacist or doctor.
Phenergan tablets contain promethazine hydrochloride, a sedating antihistamine from the phenothiazine group. They are used for allergic conditions such as hay fever and rashes, for nausea and vomiting including motion sickness, and for the short term treatment of insomnia in adults. Phenergan can be obtained from pharmacies, where a pharmacist can advise whether it is suitable for you.
Key things to know
- Phenergan tablets should be swallowed whole with water. NHS guidance advises not to chew the tablets. Check the leaflet supplied with your specific product for instructions on how to take it.
- There is no 50mg Phenergan tablet listed in the current UK product information. Promethazine tablets can be available in 10mg, 20mg and 25mg strengths, although the current branded Phenergan tablet information on the emc lists 25mg tablets.
- Phenergan should not be taken for longer than 7 days unless a doctor advises you to do so. If your symptoms do not improve or become worse after 7 days, speak to a doctor or pharmacist.
- Promethazine can cause drowsiness, which may continue into the following morning for some people. Do not drive or use machinery if you feel sleepy, dizzy or otherwise affected.
- Alcohol should be avoided while taking promethazine because it can increase its sedating effects. Other medicines that cause drowsiness may also increase the risk of sedation, so check with a pharmacist or doctor before combining them.
- Phenergan should not be used in people who are in a coma or have central nervous system depression. It should also be avoided in people who have taken a monoamine oxidase inhibitor (MAOI) within the previous 14 days.
- Age restrictions depend on the specific promethazine product and its formulation. Always check the product leaflet and speak to a pharmacist or doctor if you are unsure whether it is suitable for a child.
What is actually in the pack for motion sickness
Several questions about Phenergan start with confusion about what strengths and formulations are available in the UK. Promethazine is available in different strengths and forms, including tablets and liquid medicines, and the instructions can vary depending on the product.
For motion sickness, NHS guidance states that adults and children aged 10 years or over will usually take 25mg the night before a journey when using promethazine to prevent motion sickness. Some products may be taken 1 to 2 hours before a shorter journey instead. Always check the instructions for the specific product you have and ask a pharmacist if you are unsure.
The important point is not to assume that a higher strength will necessarily be more suitable. If you feel that Phenergan is too strong, too weak or is causing unwanted effects, do not change the dose yourself. Speak to a pharmacist or doctor for advice.
| Product | Active ingredient | Presentation | Legal category |
|---|---|---|---|
| Phenergan 10mg tablets | Promethazine hydrochloride 10mg | Pale blue film coated tablets marked PN 10, 56 per pack | Pharmacy (P) |
| Phenergan 25mg tablets | Promethazine hydrochloride 25mg | Pale blue film coated tablets marked PN 25, 56 per pack | Pharmacy (P) |
| Phenergan Elixir 5mg/5ml | Promethazine hydrochloride | Oral solution, 100ml bottle | Pharmacy (P) |
Not every pharmacy stocks every promethazine tablet strength. If you have only ever seen the 25mg pack, this may reflect local stock availability rather than the absence of a lower strength tablet. This can make the answer to one of the most common questions about Phenergan less straightforward.
Can you cut or crush promethazine tablets?
What the tablet design tells you
Phenergan tablets are film coated and do not have a score line. A score line can indicate that a tablet has been designed to be divided, but the absence of one means you should not assume that splitting the tablet will provide two equal doses.
The film coating is part of the tablet's design. It can help protect the tablet and mask its taste. Crushing or splitting a tablet that is not designed to be divided can make it difficult to know exactly how much medicine you have taken. For this reason, do not crush or split Phenergan tablets unless a pharmacist or doctor has specifically advised you to do so.
There is also no good reason to assume that crushing the tablet will make it work faster. If you are having difficulty swallowing the tablets or think the dose is not suitable, ask a pharmacist about the available options.
What to do instead
Two alternatives may be appropriate, depending on why you are taking promethazine. Both should be discussed with a pharmacist rather than changing the dose yourself.
- A lower strength promethazine tablet may be available depending on the product and local stock. If 25mg feels too strong, ask a pharmacist whether a lower strength is suitable for you.
- Liquid promethazine, such as Phenergan Elixir, is available as an oral solution. It can be measured rather than split, which may make it easier for people who have difficulty swallowing tablets. Check the product information because the recommended use and doses can differ between formulations.
One important point about the elixir is that it should not be treated as a way to experiment with your dose until you find what works. Follow the instructions for your specific product and speak to a pharmacist or prescriber before changing how much you take.
Is there a Phenergan 50mg tablet in the UK?
There is no 50mg Phenergan tablet listed in the current UK product information for the branded tablet. Promethazine products are available in different strengths and formulations, so it is important to check the specific product rather than assume that a strength mentioned online is available as a Phenergan tablet in the UK.
Some American information refers to 50mg promethazine tablets, which can cause confusion when people compare US and UK information. A medicine strength available in another country does not mean that the same strength is marketed in the UK.
The practical point is that you should not combine tablets yourself to create a higher dose. If you feel that your current dose is not working or you need a different strength, speak to a pharmacist or doctor. They can consider the reason you are taking promethazine, your other medicines and whether it is suitable for you.
Why Phenergan may not be working for sleep
If Phenergan does not seem to be helping you sleep, there can be several reasons. Understanding what is happening can help you decide whether you need advice rather than simply taking more.
The expectation may not match the medicine
Promethazine is a sedating antihistamine that can cause drowsiness. It is used for the short term treatment of insomnia in adults, but it does not treat every possible cause of sleep problems.
For example, if you fall asleep easily but regularly wake during the night, promethazine may not address the reason you are waking. Ongoing sleep problems may have another underlying cause that needs to be considered.
Repeated use may reduce the effect
The sedative effect of promethazine can become less noticeable with continued use. This is one reason the product information advises against using it for longer than 7 days without medical advice.
If you find that you need more to get the same effect, do not increase the dose yourself. Speak to a pharmacist or doctor about whether you should continue using it and what other options may be appropriate.
Timing may be off
The recommended timing depends on why you are taking promethazine and which product you are using. Taking it at the wrong time may make the medicine feel less useful or leave you feeling drowsy when you do not want to be.
Follow the instructions supplied with your medicine. If you are unsure about when to take it, ask a pharmacist rather than changing the timing or dose yourself.
If you miss a dose, follow the instructions for your specific product. Do not take a double dose to make up for a missed dose.
The problem may not be the tablet at all
Sleep can be affected by many things, including caffeine, alcohol, stress, pain, changes in routine and shift work. Other health conditions can also affect sleep.
Promethazine can cause drowsiness, and alcohol can increase this effect. It can also cause side effects such as dizziness or blurred vision in some people. If you have breathing problems or a condition affecting your lungs, speak to a doctor or pharmacist before using promethazine.
If insomnia continues for more than a short period or keeps returning, it is worth speaking to a GP rather than relying on a short term medicine. A GP can help look for possible causes and discuss suitable treatment options.
How long it takes to work and how long it lasts
A lot of online information gives very specific timings for how quickly promethazine starts working or exactly how long its effects will last. These figures can vary between people and depending on why the medicine is being used.
UK guidance advises taking promethazine at different times depending on its intended use. For sleep, the product information gives specific instructions for when to take the tablets. For motion sickness, the timing is different.
Promethazine can cause drowsiness, and some people may still feel sleepy the following day. Because the response varies from person to person, do not assume that you will be fully alert by a particular time after taking it.
If you feel sleepy, dizzy, have blurred vision or feel otherwise affected, do not drive, cycle or operate machinery. If the drowsiness is severe, persistent or concerning, speak to a pharmacist or doctor.
| Question | What the UK sources say |
|---|---|
| How quickly do symptoms improve? | NHS guidance states that symptoms usually start to improve within about 20 minutes, in the context of allergy symptoms such as itching, sneezing, and swelling, and to treat allergic conditions including allergic conjunctivitis. It is also used to treat nausea and to control nausea and vomiting, and promethazine works by blocking histamine receptors. |
| Is there a published onset time for sleep? | No. Neither the UK Summary of Product Characteristics nor NHS.uk publishes an onset figure for the sedative effect, so claims such as 30 to 60 minutes are not drawn from the product information. Slow or shallow breathing requires urgent medical attention and is not an expected effect of treatment . |
| How long does the effect last? | NHS guidance describes the medicine working for around four to six hours. The Summary of Product Characteristics states the duration of action may be up to 12 hours. |
| Why do the two differ? | Four to six hours is the useful effect most people notice. Up to 12 hours is the outer limit of the drug's action, and it is the basis for the warning about driving and machinery the next day. Promethazine should be used cautiously in people with respiratory conditions, as it may worsen some symptoms . |
Next day drowsiness, side effects, and other common problems
Drowsiness is a known effect of Phenergan because promethazine is a sedating antihistamine. Some people may still feel sleepy or less alert the following day, particularly if they take it late at night or are sensitive to its effects.
Other possible side effects include anticholinergic effects such as dry mouth, blurred vision, constipation or difficulty passing urine. The product information also lists other adverse effects for which the frequency is not known. This means there is not enough reliable information to give an exact percentage for how often they occur.
Side effects can vary from person to person. If you develop a side effect that is severe, does not go away, or concerns you, speak to a pharmacist or doctor. If you experience symptoms that could indicate a serious reaction, seek urgent medical attention.
| Problem | Why it happens | What usually helps | When to get advice |
|---|---|---|---|
| Drowsiness the next day | The duration of action may be up to 12 hours, so the effect can carry into the morning | Do not drive, cycle or use machinery while you feel drowsy, dizzy or your vision is blurred. Review timing with a pharmacist | If it happens every time, or affects your work or driving |
| Dry mouth, constipation, blurred vision | Anticholinergic effects of promethazine. Older adults are particularly susceptible | Sips of water, sugar free gum, fibre and fluids | Difficulty passing urine, severe constipation or lasting visual changes |
| Dizziness or unsteadiness | Sedation combined with effects on blood pressure | Get up slowly, avoid stairs and hot baths soon after a dose | Any fall, or unsteadiness in an older adult |
| Confusion | Older adults are more susceptible to confusion as well as anticholinergic effects | Stop and seek a review rather than continue | Promptly, especially in an older person |
| Skin reaction after sun exposure | Promethazine can cause photosensitivity, so avoid strong sunlight, UV lamps and tanning beds during treatment and shortly after | Cover skin and limit exposure | If a rash develops |
| Fever, sore throat or infection | These can be early signs of a blood disorder, including bone marrow suppression | Stop taking it | Seek same-day medical advice, as a blood test may be required |
| Stiffness, restlessness or unusual jerking | Rare movement related effects can occur because promethazine has antidopaminergic activity, including extrapyramidal symptoms and uncontrolled movements | Stop taking it and seek urgent medical advice | Urgently, especially if you also have parkinson disease or these symptoms are new |
| High fever, marked muscle rigidity and sudden severe illness | A very rare reaction called neuroleptic malignant syndrome needs urgent recognition | Stop taking it and seek emergency help | Immediately |
Two points are frequently misreported. Diarrhoea is not listed as a known side effect of Phenergan in the UK product information. Gastrointestinal effects that are listed include epigastric discomfort. If you develop persistent or severe gastrointestinal symptoms, speak to a pharmacist or doctor rather than assuming they are caused by Phenergan.
How quickly promethazine helps can also depend on why you are taking it. It may be used for allergic conditions such as rhinitis and conjunctivitis, as well as for nausea and vomiting. Promethazine may also be used in some pregnancy related situations, but this should be discussed with a healthcare professional before use during pregnancy.
There is no specific promethazine half life figure provided in the UK Phenergan product information. For this reason, be cautious with websites that give a precise number for how long the medicine stays in your system without explaining where the figure comes from.
Drug interactions worth knowing about
If Phenergan seems to affect you differently than expected, another medicine or substance could sometimes be contributing. Promethazine can increase sedation when taken with other medicines or substances that have similar effects. Some combinations can also increase the risk of breathing problems or other unwanted effects.
- Monoamine oxidase inhibitors (MAOIs). Phenergan should not be used with an MAOI or within 14 days of stopping one. If you take or have recently taken an MAOI, speak to a doctor or pharmacist before using promethazine.
- Alcohol. NHS guidance advises avoiding alcohol while taking promethazine because it can increase drowsiness and other sedative effects.
- Central nervous system (CNS) depressants. Medicines that depress the central nervous system can increase the sedative effects of promethazine. This may include some sedatives, sleeping medicines, opioids and medicines used for anxiety.
- Other sedating antihistamines. Taking promethazine with other medicines that cause drowsiness, including some antihistamines such as chlorphenamine or diphenhydramine, may increase sedation. Some cold and flu products contain these ingredients, so check the label before taking them together.
- Other medicines, supplements and herbal products. Tell your doctor or pharmacist about all the medicines and products you take, including those bought without a prescription, before using promethazine.
- Medicines that can affect heart rhythm. Some medicines that prolong the QT interval may interact with promethazine. These can include certain antiarrhythmics, antimicrobials, antidepressants and antipsychotics. Your doctor or pharmacist can check whether your medicines are suitable to take together.
- Amitriptyline and other medicines affected by CYP2D6. Promethazine can affect the metabolism of some medicines, including amitriptyline. If you take these medicines together, a healthcare professional may need to review your treatment.
- Allergy skin testing. Promethazine can affect the results of allergy skin tests by reducing the skin's response to histamine. It may need to be stopped before testing. Ask the clinic performing the test how long you should stop it beforehand.
- Diagnostic tests. Tell the healthcare professional carrying out a test that you are taking promethazine. This allows them to consider whether the medicine could affect the result or the interpretation of the test.
Promethazine is not known to reduce the effectiveness of hormonal or non hormonal contraception. If you are concerned about a particular medicine or contraceptive, a pharmacist can check whether there is an interaction.
Mistakes that turn a minor issue into a complication
| The mistake | Why it matters |
|---|---|
| Taking a second dose because the first did not work | NHS guidance is never to take two doses at once, and never to take an extra to make up for a missed one |
| Adding another drowsy antihistamine | Doubling the sedation with no added benefit for sleep |
| Drinking alcohol the same evening | Deep sedation, impaired breathing and difficulty waking |
| Using it every night for weeks | Tolerance develops, and stopping after prolonged regular use may lead to symptoms such as dizziness, sweating, nausea, palpitations and rebound insomnia |
| Splitting or crushing tablets | No score line and a film coating make the amount you take unpredictable |
| Treating the elixir as a self adjustment tool | Liquid is easy to vary, which is precisely why changes belong with a pharmacist |
| Giving it to a young child | Phenergan is contraindicated in children under six years of age |
Why people think Phenergan has been discontinued
Phenergan has not been discontinued in the UK. Phenergan tablets and oral liquid remain available as pharmacy medicines, although the specific strengths and formulations available can vary between pharmacies and suppliers. This can sometimes make the medicine appear to be unavailable when a particular product is temporarily out of stock.
The confusion can come from several sources:
- Different availability in the United States. Information from the US can appear prominently in online searches, but the availability and licensing of promethazine products can differ between the US and UK.
- Older Phenergan products. Some people may remember combination products containing Phenergan that are no longer available in the UK. This does not mean that all Phenergan products have been discontinued.
- Local stock shortages. A pharmacy may not have a particular strength or formulation in stock at a given time. This is different from the medicine being discontinued nationally.
It is also worth separating availability from how the medicine is used. Healthcare professionals may be cautious about using sedating antihistamines for long term sleep problems because of concerns such as next day drowsiness and reduced effectiveness with continued use. This is different from Phenergan being withdrawn from the UK market.
When to stop and get advice
Speak to a pharmacist or GP rather than changing your dose or stopping and restarting treatment without advice if you experience any of the following:
- Persistent or severe drowsiness, especially if it continues into the following day or is accompanied by confusion.
- Fever, sore throat or signs of infection. Rare blood related problems have been reported with promethazine. Seek medical advice promptly if you develop these symptoms.
- A fast, slow or irregular heartbeat, difficulty passing urine, or significant changes in your vision.
- A rash, swelling of the face, lips, mouth or throat, or difficulty breathing. These may be signs of a serious allergic reaction and require urgent medical attention.
- Ongoing sleep problems. If insomnia continues for more than a few weeks or you regularly need medicine to fall asleep, speak to your GP rather than continuing to use a short term treatment without medical advice.
If you have taken more than the recommended amount of promethazine, seek urgent medical advice. In the UK, you can contact NHS 111 for advice or go to an emergency department if you have concerning symptoms. Take the medicine packaging with you if possible.
An overdose can cause symptoms such as severe drowsiness, agitation, confusion, a very dry mouth, unsteadiness and a fast heartbeat. More serious poisoning can cause seizures, loss of consciousness or other serious effects. Do not wait for symptoms to develop if you think you have taken too much. Seek medical advice promptly.
Buying Phenergan in the UK
Phenergan is a pharmacy medicine rather than a general sales product. It can be bought without a prescription, but only through a pharmacy and only after a suitability check covering your symptoms, your health and your other medicines. A registered online pharmacy carries out the same check.
| Product | Pack size | Price |
|---|---|---|
| Phenergan 25mg tablets (promethazine hydrochloride) | 56 tablets | £14.99 |
| Phenergan Elixir 5mg/5ml oral solution | 100ml | £14.49 |
Prices were correct at the time of writing but should be confirmed before purchase. Availability of individual strengths can also vary between pharmacies and suppliers. Wherever you buy Phenergan or another promethazine product, check that the pharmacy is registered with the General Pharmaceutical Council (GPhC). A legitimate online pharmacy should display its registration details, identify its superintendent pharmacist where applicable, and provide a clear way for you to contact the pharmacy if you have questions about your medicine.
For information about possible side effects rather than the practical problems covered in this guide, see our detailed guide to Phenergan side effects when used for sleep.
If you are unsure whether promethazine is suitable for you because of your age, health conditions or other medicines, see our guide to who can and cannot take promethazine.
Storage is also important, particularly for the elixir, which has a shorter shelf life after opening than the tablets. Our guide on how to store promethazine safely explains how to store both forms.
If you have questions about your individual circumstances, speak to a pharmacist. You can also contact our clinical team for further information.
Medical Disclaimer
This article provides general information and does not replace advice from a qualified healthcare professional. It does not provide individual dosing advice, and the information should not be used to start, stop or change how you take any medicine without professional advice.
Whether promethazine is suitable depends on individual factors, including your age, medical history, pregnancy or breastfeeding status and any other medicines you take. Effects and side effects can vary between individuals.
Always read the patient information leaflet supplied with your medicine. If you are unwell or unsure about your treatment, speak to a pharmacist, your GP or NHS 111. If you have a medical emergency, call 999.
The information in this article is based on the UK Summary of Product Characteristics and Patient Information Leaflet for Phenergan tablets and elixir available through the electronic Medicines Compendium (emc), together with NHS guidance on promethazine. Information should be checked against the latest available guidance and product information before use.
