A sedating (older) antihistamine
Diphenhydramine
An older, sedating antihistamine that causes drowsiness — used in the UK mainly as a sleep aid (Nytol) rather than for allergy.
What is Diphenhydramine?
Diphenhydramine is an older, first-generation antihistamine that commonly causes drowsiness. It is used for allergy symptoms and itching, and because of its sedating effect it is also found in some short-term sleep aids and cough and cold remedies.
Education and reference only. This is a plain-language guide to Diphenhydramine — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Diphenhydramine is an older, "first-generation" antihistamine. Unlike the modern non-drowsy antihistamines, it makes most people feel sleepy, and this is exactly why it is most commonly used as a short-term sleep aid for occasional insomnia. There is an important UK/US difference to be aware of: in the US, diphenhydramine is the well-known "Benadryl" allergy product, but in the UK the "Benadryl" brand contains a different antihistamine altogether (such as acrivastine or cetirizine). So in the UK, diphenhydramine is sold mainly as the sleep aid Nytol, not as an allergy medicine. It is available over the counter for short-term use.
How it works
Like other antihistamines, diphenhydramine blocks histamine (H1) receptors, which can relieve allergy symptoms. But because it readily crosses into the brain, it also blocks histamine's wakefulness-promoting effect there, which is what makes it sedating. This drowsiness — a side effect for an allergy medicine — is the very reason it is used to help people fall asleep. It also has "anticholinergic" actions, blocking another messenger called acetylcholine, which causes effects such as a dry mouth and is why it needs care in older people.
Company & origin
Originated / developed by: Parke-Davis (now Pfizer).
Diphenhydramine was discovered in 1943 by chemist George Rieveschl at the University of Cincinnati and developed commercially by Parke-Davis. It became the first antihistamine approved by the FDA, in 1946, and was marketed as Benadryl.
What it treats
Conditions Diphenhydramine is used for
Practical use
How to take Diphenhydramine
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Take it as directed; because it causes drowsiness, it is often taken in the evening.
- Do not drive or operate machinery until you know how it affects you.
- Avoid alcohol, which adds to the drowsiness.
- Check other cold, cough or sleep remedies you take, as some also contain antihistamines.
- Use it only for short periods unless advised, and take particular care in older people.
Weighing it up
Advantages & disadvantages of Diphenhydramine
Advantages
- Relieves allergy symptoms and itching.
- Its sedating effect can help when used short-term for sleep.
- Widely available without prescription.
- Long-established and familiar.
Disadvantages
- Commonly causes drowsiness, which can affect driving and daily activities.
- Can cause dry mouth, blurred vision, constipation and difficulty passing urine.
- More likely to cause confusion and falls in older people, so it is generally avoided in the elderly.
- The body can become less responsive to its sleep effect with repeated use.
- Modern non-drowsy antihistamines are usually preferred for everyday allergy relief.
Practical use
Good to know
In the UK it is most often used short-term to help with occasional sleeplessness, taken before bed; it is not meant for long-term use, as the body gets used to it and it stops helping. Because it is sedating, you should not drive or operate machinery if you feel drowsy, and the next-morning "hangover" effect is common. Avoid alcohol with it. Its anticholinergic effects — dry mouth, blurred vision, constipation and confusion — make it a poor choice in older people, who are more sensitive to these and to the risk of falls. Remember the UK/US naming difference: UK shoppers wanting diphenhydramine should look for Nytol, not Benadryl.
Who should not take it / use with caution
- Older people, in whom sedating antihistamines can cause confusion, falls and other anticholinergic effects — they are best avoided.
- People with certain conditions worsened by anticholinergic effects, such as some types of glaucoma, an enlarged prostate with difficulty passing urine, or certain bowel problems.
- Used with caution in significant liver or kidney impairment, in epilepsy, and is not intended for long-term use.
Monitoring
- Generally none for short-term use
- Review if used repeatedly for sleep
- Particular caution and review in older people
Side effects
- Drowsiness — the main effect, and the reason it is used for sleep — along with a "hangover" grogginess the next morning.
- Anticholinergic effects: dry mouth, blurred vision, constipation and difficulty passing urine.
- Dizziness and, especially in older people, confusion and an increased risk of falls.
Key interactions
- Other sedating medicines and alcohol add to the drowsiness and grogginess.
- Other medicines with anticholinergic effects add up, increasing dry mouth, constipation and confusion.
- Tell your pharmacist what else you take, including sleep, allergy and cold remedies, as several contain sedating antihistamines.
Available as: Tablets and capsules, and a liquid; in the UK most often sold as a sleep aid (e.g. Nytol) for short-term use.
Answers
Diphenhydramine: frequently asked questions
Why does diphenhydramine make me sleepy when newer antihistamines don't?
Diphenhydramine is an older "first-generation" antihistamine that crosses easily into the brain, where it blocks histamine's wakefulness signal — so it causes drowsiness. Newer antihistamines like loratadine and cetirizine were designed not to do this, which is why they are "non-drowsy".
In the UK, is Benadryl the same as diphenhydramine?
No — and this is a common source of confusion. In the US, Benadryl is diphenhydramine, but in the UK the Benadryl brand contains a different antihistamine (such as acrivastine or cetirizine). In the UK, diphenhydramine is sold mainly as the sleep aid Nytol. So if you want diphenhydramine in the UK, look for Nytol, not Benadryl.
Can I use diphenhydramine to help me sleep every night?
No — it is only intended for short-term, occasional sleeplessness. The body quickly gets used to it so it stops working well, and regular use brings next-day grogginess and other effects. If you have ongoing sleep problems, see a pharmacist or GP about longer-term approaches.
Is diphenhydramine safe for older people?
It is generally best avoided in older people. Sedating antihistamines like diphenhydramine have "anticholinergic" effects that can cause confusion, dry mouth, constipation and a higher risk of falls, and older people are more sensitive to these. A pharmacist can suggest safer alternatives.
What is the difference between diphenhydramine and Nytol or Benadryl?
Diphenhydramine is the generic (active-ingredient) name. In the UK it is sold as the sleep aid Nytol, and in the US it is the allergy brand Benadryl — but note that UK "Benadryl" is a different antihistamine entirely. Where a product genuinely contains diphenhydramine, the active ingredient is identical regardless of brand.
The wider class
About Antihistamines
Diphenhydramine belongs to the antihistamines class. For how the class as a whole works, its shared safety principles and monitoring, see the full guide.
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Authoritative sources
- BNF: Diphenhydramine hydrochloride.
- NICE CKS: Insomnia.
- NICE CKS: Sleeping pills and antihistamines.
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