1. How long does Phenergan 25mg take to work for sleep?
Phenergan usually starts to make you feel drowsy within around 30 minutes to 1 hour after taking it. It should be taken at bedtime when you are able to have a full night’s sleep.
What are Phenergan 25mg Tablets?
Phenergan 25mg Tablets is a medication used to treat short-term sleep disturbances and allergic conditions. The active ingredient is promethazine hydrochloride, which is a sedating antihistamine.
Promethazine works by blocking histamine in the body, helping to reduce allergic symptoms and causing drowsiness, which can help you sleep.
Who is Phenergan suitable for?
This medication may be suitable for adults and children over the age of 16 who are experiencing:
- Short-term sleep disturbances
- Allergic conditions such as hay fever or rashes
- Night-time itching caused by allergies
However, if your sleep does not improve, you should contact your GP to identify the underlying cause.
Do not take this medication if you are allergic to promethazine or any of the ingredients.
This medication may not be suitable if you:
- Are under 16 years old
- Are pregnant or breastfeeding
- Have breathing problems such as asthma
- Have glaucoma
- Have prostate problems or difficulty passing urine
How to take Phenergan 25mg Tablets
You should always take this medication as advised by your GP or pharmacist, and always refer to the patient information leaflet.
Adults and children aged 16+:
Take ONE tablet at night.
Phenergan should be taken at bedtime due to its sedating effects.
- Do not take more than the recommended dose
- Do not take additional doses during the night
- Avoid alcohol while taking this medication
- Only take if you can have a full night’s sleep (at least 8 hours)
How to store
- Store in a cool, dry place
- Keep below 25°C
- Keep in the original packaging
- Keep out of sight and reach of children
- Do not use after the expiry date on the packaging
The most common side effects experienced when taking Phenergan include:
- Drowsiness
- Dizziness
- Dry mouth
- Blurred vision
In rare cases, Phenergan can cause an allergic reaction.
Symptoms of an allergic reaction include:
- A rash
- Hives
- Swelling of the face, mouth, tongue or throat
If you suspect you are having an allergic reaction, seek emergency help immediately.
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Frequently Asked Questions
2. Can I feel drowsy the next day after taking Phenergan 25mg?
Yes, some people may still feel sleepy or less alert the next day after taking Phenergan. If this happens, do not drive, cycle, or operate machinery until you feel fully alert again.
3. Do I need a prescription for Phenergan 25mg Tablets?
No, Phenergan 25mg Tablets do not usually require a prescription, but they are a pharmacy-only medicine. This means a pharmacist will need to make sure the product is suitable for you before it is supplied.
Our Clinical Authors
Written by
Omar El-Gohary
CEO & Superintendent Pharmacist | GPhC Registration: 2059792
Medically reviewed by
Jabbar Ali
MPharm Clinical Lead | GPhC Registration: 2205490
This page has been written by a registered UK pharmacist and reviewed by our clinical team to ensure it is accurate, clear and reflects current clinical guidance.
Last medically reviewed: 20 August 2026
Last updated: 21 August 2026