1. How long does Sominex take to work?
Sominex usually starts to work within 30 to 60 minutes. It should be taken at bedtime when you are ready to sleep.
What is Sominex?
Sominex is a medication used to treat temporary, short-term sleep disturbances. The active ingredient in Sominex is promethazine hydrochloride (20mg), which is a sedating antihistamine.
Promethazine works by blocking histamine in the body, helping to cause drowsiness and promote sleep.
Who is Sominex suitable for?
This medication may be suitable for adults who are experiencing short-term sleep disturbances caused by:
Patients may find it easier to fall asleep and stay asleep for longer.
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 Sominex
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 bedtime.
- Do not take more than the recommended dose
- Do not take an extra dose 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 Sominex
- 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 Sominex include:
- Drowsiness
- Dizziness
- Dry mouth
- Blurred vision
In rare cases, Sominex 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 Sominex?
Yes, Sominex can cause next-day drowsiness in some people. If you feel sleepy or less alert, do not drive or operate machinery until you feel fully alert again.
3. Do I need a prescription for Sominex?
No, Sominex does not require a prescription, but it is a pharmacy-only medicine. A pharmacist will check that it is suitable for you before it is supplied.
Our Clinical Authors
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: 06 July 2026
Last updated: 06 July 2026