Most problems people have with promethazine fall into two groups. Either it causes more effects than they expected, such as dry mouth, constipation or next day drowsiness, or it does not seem to work as expected. Neither situation is usually a reason to change the dose yourself.
This guide explains common anticholinergic side effects, why they happen, what to check if promethazine does not seem to be working, who should not take it, and how long it should be used. It also looks at two common questions about promethazine: whether it can be used long term and whether it helps with anxiety.
Promethazine is a drowsy antihistamine with antiemetic and anticholinergic effects. It can also cause more serious side effects in some people, including breathing problems, abnormal heart rhythms and movement disorders. These are uncommon or have a frequency that cannot be estimated from the available data, but they are important to know about.
Some people need extra care when taking promethazine, including people with epilepsy, certain heart or lung conditions, liver or kidney problems, glaucoma, or an enlarged prostate. It can also interact with alcohol and other medicines that cause drowsiness.
Taking the right dose at the right time is important. Do not increase the dose if it does not seem to be working, and do not take another dose sooner than the instructions allow. Promethazine can also cause increased sensitivity to sunlight, so strong sunlight and ultraviolet light should be avoided during and shortly after treatment.
If you develop severe drowsiness, breathing problems, unusual movements, severe agitation, or other worrying symptoms, seek medical advice promptly.
Key things to know
- Sedation or sleepiness is classed as very common with Phenergan. Many other side effects are listed as having a frequency that cannot be estimated from the available data.
- Dry mouth and constipation are recognised anticholinergic side effects of promethazine.
- Phenergan should not be used for longer than 7 days without seeking medical advice.
- It should not be taken if you are taking an MAOI antidepressant or have stopped one within the previous 14 days.
- Phenergan should not be given to children under 6 years of age.
- Promethazine is not licensed to treat anxiety. Feeling sleepy after taking it is not the same as treating anxiety.
- The duration of action can be up to 12 hours, so drowsiness can continue into the following day.
Phenergan dry mouth
Does Phenergan cause dry mouth? Yes. Dry mouth is a recognised side effect of promethazine and is linked to its anticholinergic effects.
Promethazine affects acetylcholine as well as histamine. Acetylcholine is involved in the production of saliva, so blocking some of its effects can leave your mouth feeling dry.
Drinking water can help, but it may not completely get rid of the dryness. Try taking small sips regularly rather than drinking a large amount at once. Sugar free chewing gum or sweets can also help stimulate saliva.
It is also worth paying attention to your teeth. Having a dry mouth can make dental problems more likely over time, so keep brushing your teeth regularly and avoid having sugary drinks or sweets throughout the day.
If your dry mouth is severe, makes eating or speaking difficult, or continues after you stop taking promethazine, speak to a pharmacist or doctor. They can suggest ways to manage it and check whether another medicine would be more suitable.
Phenergan constipation
Can Phenergan cause constipation? Yes. Constipation is a recognised anticholinergic side effect of promethazine.
Promethazine can affect the normal movement of the bowel, which can make stools harder to pass. If you notice constipation while taking it, simple measures may help. Drink enough fluid, eat fibre rich foods and keep moving rather than sitting for long periods. Do not ignore the urge to open your bowels.
If constipation becomes severe, keeps getting worse, or happens with significant abdominal pain, swelling or vomiting, seek medical advice rather than simply taking more laxatives or continuing the medicine without advice.
Why these effects happen together
It can help to look at these side effects as a group rather than treating each one as a separate problem. Promethazine is an antihistamine with additional anticholinergic effects. These can contribute to dry mouth, constipation, difficulty passing urine, blurred vision and problems with focusing.
| Effect | Why it happens | What may help |
|---|---|---|
| Dry mouth | Reduced saliva production | Sip water, use sugar free gum and keep up good dental care |
| Constipation | Slower bowel movement | Drink enough fluid, eat fibre rich foods and stay active |
| Blurred vision | Anticholinergic effects can affect focusing | Do not drive while your vision is affected |
| Difficulty passing urine | Anticholinergic effects can affect bladder function | Speak to a pharmacist or doctor |
| Drowsiness | Promethazine blocks histamine H1 receptors in the brain | Take it as directed and do not drive if you feel sleepy |
If you have several of these effects at the same time, they may be related to the medicine rather than being separate problems. Older adults can be more sensitive to the anticholinergic effects of promethazine and may be more likely to experience confusion.
Promethazine can also cause photosensitivity. Avoid strong sunlight and ultraviolet light during and shortly after treatment, particularly if you notice that your skin becomes unusually sensitive.
Promethazine not working, and not making you sleepy
If you have searched for promethazine not working, why is my promethazine not working, Phenergan not working, or promethazine not making me sleepy, there are several possible reasons.
If you are using promethazine for sleep, one possibility is that its sedating effect becomes less noticeable when it is used repeatedly. This is one reason it is intended for short term use rather than as a regular long term sleep aid.
Promethazine also does not make everyone feel strongly sleepy. Some people can experience agitation, restlessness or other unusual reactions instead of the expected drowsiness.
If it worked at first and then seems to have stopped working, do not increase the dose yourself. Think about what has changed, including your sleep routine, other medicines, alcohol or caffeine intake, or the reason you are having difficulty sleeping.
Promethazine can make you drowsy, but it does not treat every possible cause of poor sleep. If you regularly need something to help you sleep, it is better to speak to a GP or pharmacist about the underlying problem.
Promethazine not working for nausea
Promethazine is licensed for preventing and treating motion sickness, as well as for some other types of nausea and vomiting. The right medicine can depend on what is causing your symptoms.
For travel sickness, timing matters. When used to prevent motion sickness, promethazine is usually taken before the journey. For adults and children aged 10 years and over, NHS guidance says 25mg is usually taken the night before the journey, although some products are taken 1 to 2 hours before a short journey. Follow the instructions for your particular product rather than changing the timing yourself.
If you are already feeling sick or vomiting, taking an oral medicine may be more difficult. If promethazine is not helping, or your nausea is not related to travel, speak to a pharmacist or doctor rather than taking extra doses.
Melatonin and Sleep Timing
Melatonin is a hormone made naturally by the body that helps control the sleep wake cycle. It is different from promethazine because it is linked to the body's sleep timing rather than simply causing drowsiness.
Melatonin may be used in some circumstances for sleep timing problems, such as jet lag, but it is not suitable for everyone and is not a general replacement for promethazine. If your main problem is difficulty sleeping, speak to a GP or pharmacist about the cause and the most suitable treatment.
Improving Sleep Quality
Better sleep usually involves more than taking a medicine. Keeping a regular sleep schedule, getting regular physical activity, limiting caffeine and alcohol, and making your bedroom comfortable can all help.
If sleep problems continue, it is worth looking at what may be keeping you awake rather than relying on a sedating medicine every night. Cognitive behavioural therapy for insomnia, known as CBT I, can help with the thoughts and habits that contribute to ongoing insomnia. NHS guidance recommends speaking to a GP if changing your sleep habits has not helped or if sleep problems are affecting your daily life.
How long does Phenergan take to work, and the hangover afterwards
There is no specific onset time stated in the Phenergan product information for when drowsiness begins, so it is better not to rely on an exact number of minutes.
What is clear is that Phenergan can have a long duration of action, with product information stating that it can last for up to 12 hours. This means a dose taken at night can still cause drowsiness the following morning.
People sometimes describe this as a Phenergan hangover. It is not an alcohol hangover. It refers to the lingering effects of the medicine, such as sleepiness, dizziness or difficulty concentrating.
If you feel drowsy, dizzy, have blurred vision, feel clumsy or cannot concentrate properly, do not drive, cycle or operate machinery. Speak to a pharmacist if these effects are making it difficult to manage your normal activities.
Phenergan restless legs and agitation
Can Phenergan cause restless legs? It can. Restless legs syndrome is listed as a possible nervous system side effect of promethazine, and the medicine can also cause restlessness or agitation.
This can feel like an uncomfortable need to keep moving, which can be confused with restless legs syndrome. Other movement related effects, including muscle spasms, unusual facial movements and other extrapyramidal effects, are also listed in the product information.
If you develop new restlessness, unusual movements or severe agitation after taking promethazine, do not increase the dose. Speak to a pharmacist or doctor for advice.
Phenergan contraindications
Some people should not take Phenergan. The main contraindications for the oral Phenergan products include:
- A known allergy to promethazine, other phenothiazines, or any of the ingredients in the medicine.
- Coma or central nervous system depression from any cause.
- Children under 6 years of age.
- Taking an MAOI antidepressant, or having stopped one within the previous 14 days.
There are also several conditions where Phenergan should be avoided or used with particular caution. These include liver or kidney problems, Parkinson's disease, an underactive thyroid, heart failure, myasthenia gravis, an enlarged prostate, narrow angle glaucoma, asthma, bronchitis, bronchiectasis and epilepsy.
Promethazine can also interact with other medicines, particularly medicines that cause drowsiness or have anticholinergic effects. It can also add to the effects of alcohol. Tell your pharmacist or doctor about all the medicines you take before using it.
Long term use of Phenergan
Phenergan is not intended to be used long term without medical advice.
For the oral Phenergan products, the product information states that it should not be used for longer than 7 days without seeking medical advice. If you find that you need it most nights for sleep, that is a good reason to speak to a GP rather than continuing to take it on your own.
Repeated use can also become less useful for sleep because the sedating effect may become less noticeable. Older adults need particular care because they can be more sensitive to the anticholinergic effects of promethazine, including confusion.
If you have had sleep problems for weeks or months, the better approach is to look for the underlying cause. NHS guidance recommends seeking medical advice when insomnia continues or affects your daily life. CBT can be one of the treatments used for ongoing insomnia.
Phenergan anxiety
Promethazine is not licensed to treat anxiety in the UK. Its licensed uses include allergic conditions, motion sickness, nausea and vomiting, short term treatment of insomnia in adults, and certain sedative uses.
It can make you feel sleepy, but that does not mean it is treating the cause of anxiety. Some people can also experience agitation or anxiety as a side effect.
If anxiety is affecting your everyday life, speak to a GP. There are treatments specifically aimed at anxiety, including talking therapies such as CBT, depending on the cause and your individual circumstances.
Phenergan reviews, and what to make of them
Phenergan reviews can be useful for seeing what other people experienced, but they are not a reliable way to decide whether the medicine is right for you.
One person may say it made them very sleepy, while another may say it did not make much difference. Both experiences can happen, and online reviews usually do not include important details such as the dose taken, why the medicine was being used, or what other medicines the person was taking.
It is more useful to check what Phenergan is licensed to treat, whether it is suitable for you, and whether there are any medicines or health conditions that could affect its use. A pharmacist can help you put those details together.
What not to do when something is wrong
- Do not increase the dose because Phenergan does not seem to be working. Taking more can increase the risk of side effects and overdose.
- Do not split Phenergan tablets unless a pharmacist or doctor has specifically told you that the particular tablet can be split. The 25mg Phenergan tablets are film coated and do not have a score line.
- Do not switch between tablets and elixir to create your own dose. The strengths and instructions can differ between formulations.
- Do not take another drowsy antihistamine with promethazine unless a doctor has advised you to do so. This can increase drowsiness and other side effects.
- Do not continue using Phenergan for longer than 7 days without seeking medical advice.
- Do not drink alcohol while taking promethazine. Alcohol can increase its sedating effects and make you very sleepy.
- Do not take another dose earlier than instructed. If you are unsure when your next dose is due, ask a pharmacist.
When to get medical advice
Speak to a pharmacist or doctor if you have side effects that are troublesome, do not go away, or are affecting your normal activities. Some symptoms need more urgent attention.
- Severe constipation with significant abdominal pain, swelling or vomiting, particularly if you cannot pass stools or wind.
- Difficulty passing urine, especially if it is new or painful.
- Severe confusion, unusual agitation or marked changes in behaviour.
- New restlessness or an inability to keep still after taking promethazine.
- Unusual muscle stiffness, tremors, spasms or involuntary movements.
- Severe drowsiness, particularly if you are difficult to wake or your breathing is affected.
- Difficulty breathing, swelling of the face, lips, tongue or throat, or other signs of a serious allergic reaction. Seek emergency medical help if this happens.
- A fast, irregular or pounding heartbeat, particularly after taking more than the recommended dose.
- Symptoms of severe overheating, muscle stiffness, confusion or changes in consciousness. These can be signs of a rare but serious reaction called neuroleptic malignant syndrome and need urgent medical attention.
- Persistent symptoms that are not improving, particularly if you have been using Phenergan for sleep and are approaching 7 days of use.
- If you take other medicines that cause drowsiness, affect the heart rhythm, lower the seizure threshold, or have anticholinergic effects, ask a pharmacist or doctor whether they can be taken with promethazine.
- If you feel drowsy, dizzy or unable to concentrate after taking promethazine, do not drive or operate machinery until you feel fully able to do so.
Suspected side effects can be reported through the MHRA Yellow Card Scheme. These reports help regulators monitor the safety of medicines after they are licensed.
