# Nytol and other sleep antihistamines: do they work?

Source: https://sleepfacts.uk/sleep-aids/antihistamines/
Author: Tess Marlow · Updated 2026-11-04

**Short answer:** Sedating antihistamines such as diphenhydramine (Nytol Original, Boots Sleepeaze) and promethazine (Sominex, Phenergan) make you drowsy, but trials show only small effects on sleep, and the body gets used to them quickly. They can leave you groggy the next day. The NHS says to speak to a doctor if you need them for more than 2 weeks.

**Evidence grade:** Weak evidence. Small or short studies, surveys, or results that are mixed.

## What are sleep antihistamines?

Antihistamines block histamine, a chemical involved in allergies. Histamine is also one of the brain's signals for staying awake, so older antihistamines that reach the brain make people drowsy. Two are sold in UK pharmacies for short-term sleep problems:

| Medicine | UK brands | Sold for sleep as |
| --- | --- | --- |
| Diphenhydramine | Nytol Original, Boots Sleepeaze | Tablets and liquid, 16 and over |
| Promethazine | Sominex, Phenergan | Tablets, also used for allergies and travel sickness |

Some night-time cold and flu remedies also contain them, so check the label before taking a second product.

## Do they work?

They make people sleepy, but the effect on sleep itself is small. In a US trial of 184 adults with mild insomnia, diphenhydramine (50 mg a night) for 14 days increased sleep efficiency compared with placebo, but most other measures didn't differ, and sleep monitoring in the lab showed no clear changes. US sleep medicine guidelines advise against diphenhydramine for chronic insomnia because the benefit is small and the evidence is weak.

The bigger problem is that the brain gets used to them. The NHS warns that diphenhydramine may stop working as well if taken for too long without a break, and that people can become dependent on it.

## What are the side effects?

- **Next-day drowsiness.** The NHS says diphenhydramine's drowsiness usually lasts about 8 hours. Don't drive, cycle or use machinery if you feel sleepy.
- **Dry mouth, dizziness, unsteadiness and difficulty concentrating.**
- **Constipation and difficulty passing urine,** especially in older men with an enlarged prostate.
- **Confusion, in older people.** These medicines block acetylcholine, a brain chemical important for memory, and older people are more sensitive to that.

Don't drink alcohol while taking them, because it increases the side effects. Don't combine them with other sedatives or sleeping pills.

## Who should avoid them?

Ask a pharmacist or GP first if you are over 65, pregnant or breastfeeding, or have glaucoma, an enlarged prostate, epilepsy, or liver or kidney problems, or if you take other medicines that cause drowsiness.

## When might they make sense?

For a night or two when something specific is keeping you up, such as an itchy rash, a cold or a stressful event, a sedating antihistamine is a reasonable short-term option for a healthy adult. For sleep problems that keep coming back, they don't address the cause. See [insomnia](/sleep-problems/insomnia/) and [CBT for insomnia](/sleep-problems/cbt-for-insomnia/).

## Frequently asked questions

### Is it OK to take Nytol every night?

Not for long. The NHS says you can become dependent on diphenhydramine if you take it for too long without a break, and it may stop working as well. Speak to your doctor if you need it for more than 2 weeks.

### Why do I feel groggy after taking Nytol?

Diphenhydramine's drowsiness usually wears off about 8 hours after a dose, according to the NHS, so if you take it late or sleep for less than 8 hours you may still feel the effects in the morning. Don't drive or use machinery if you feel drowsy.

### Are sleep antihistamines safe for older people?

Older people are more likely to get side effects, including confusion, dizziness, constipation and difficulty passing urine, and drowsiness raises the risk of falls. If you are over 65, speak to a pharmacist or GP before taking them.

### Do non-drowsy hay fever tablets help sleep?

No. Antihistamines such as cetirizine, loratadine and fexofenadine are designed not to reach the brain much, so they don't usually cause sleepiness.

## References

1. [NHS. Diphenhydramine.](https://www.nhs.uk/medicines/diphenhydramine/)
2. [NHS. Promethazine (Phenergan).](https://www.nhs.uk/medicines/promethazine/)
3. [Morin CM et al. Valerian-hops combination and diphenhydramine for treating insomnia: a randomized placebo-controlled clinical trial. Sleep, 2005.](https://pubmed.ncbi.nlm.nih.gov/16335333/)
4. [Sateia MJ et al. Clinical practice guideline for the pharmacologic treatment of chronic insomnia in adults. Journal of Clinical Sleep Medicine, 2017.](https://pubmed.ncbi.nlm.nih.gov/27998379/)