Diphenhydramine is an antihistamine medicine that relieves the symptoms of allergies. It's known as a drowsy (sedating) antihistamine and is more likely to make you feel sleepy than other antihistamines. It's used for:

• short-term sleep problems (insomnia), including when a cough or cold, or itching, is keeping you awake at night - brand names include Nytol Original, Nytol One-a-Night and Sleepeaze
• cough and cold symptoms - brand names include Benylin Chesty Coughs and Covonia Night Time Formula
• hay fever - brand names include Histergan
• eczema, hives (urticaria), insect bites and stings - brand names include Histergan

You can buy diphenhydramine from pharmacies and supermarkets. Diphenhydramine is also available on prescription.

It comes as tablets, capsules and a liquid that you swallow. For skin allergies like hives or bites and stings it's also available as a cream. The cream is much less likely to make you feel sleepy than the tablets, capsules or liquid.

You can also buy it mixed with other medicines, such as levomenthol, paracetamol, pholcodine and pseudoephedrine, to treat cough and cold symptoms.

• Avoid drinking alcohol while taking diphenhydramine. Alcohol increases the risk of side effects.
• To help you sleep, you'll usually take your medicine 20 minutes before you go to bed. It normally takes about 30 minutes to work.
• Common side effects include feeling sleepy, dizzy or unsteady on your feet. You may also have difficulty concentrating and a dry mouth.
• Diphenhydramine is also called by the brand names Histergan, Nytol Original, Nytol One-A-Night and Sleepeaze.
• When it's mixed with other medicines, brand names include Benylin Chesty Coughs, Benylin Children’s Night Coughs, Covonia Night Time Formula, Panadol Night Pain and Unicough.

Diphenhydramine can be taken by most adults and young people aged 16 years and above. You can sometimes give diphenhydramine to children, depending on their age and their symptoms. Ask your pharmacist for advice.

Diphenhydramine isn't suitable for some people. Tell your doctor or pharmacist if you:

• have had an allergic reaction to diphenhydramine or any other medicines in the past
• have lung problems, such as asthma or COPD, or an illness that creates a lot of phlegm
• have an eye problem called primary angle closure glaucoma
• have a stomach ulcer, or a blockage in your stomach or gut
• have kidney or liver problems
• have problems peeing or emptying your bladder
• have epilepsy or any other health problem that causes fits
• have an intolerance to, or cannot absorb, some sugars such as lactose or sorbitol
• are due to have an allergy test - diphenhydramine can affect your results, so you may need to stop taking it a few days before your test - ask a pharmacist or your doctor for advice
• are unable to have any alcohol - some liquid diphenhydramine products contain a small amount of alcohol, so check the ingredients and the packaging carefully

Giving diphenhydramine to children

For some symptoms, you can give diphenhydramine to children over the age of 6 years. Check with your pharmacist or doctor to make sure whether a product is suitable for your child.

Children aged 6 years and above can have diphenhydramine for:

• insect bites and stings - diphenhydramine cream only
• eczema - diphenhydramine cream only
• cough and cold symptoms (this doesn't apply to all cough and cold medicines that contain diphenhydramine, so check the instructions that come with the packaging carefully)

Do not give children diphenhydramine for hay fever or other allergies until they are 12 years old or above.

If you or your child have been prescribed diphenhydramine, follow your doctor's instructions about how and when to take it.

Only take or use diphenhydramine when you need it - for example if you're unable to sleep because you're worrying about something or your cold symptoms are keeping you awake.

The instructions will be different, depending on the type of diphenhydramine you buy - and the other medicines it's mixed with.

How much to use

The usual dose depends on the type of diphenhydramine you're taking, and what you're taking it for.

If you're aged over 65, start with the lower dose, as you may be more likely to get side effects.

• Short-term insomnia - diphenhydramine comes as 25mg and 50mg tablets. You'll usually take 50mg, 20 minutes before you go to bed.
• Cough and cold symptoms - diphenhydramine comes mixed with other medicines to treat the symptoms of coughs and colds. It comes as tablets, capsules and as a liquid that you swallow. The usual dose depends on the type of medicine you're taking. Check the instructions on the packaging carefully, or ask your doctor or pharmacist if you're unsure.
• Hay fever and allergies - diphenhydramine comes as 25mg and 50mg tablets. The usual dose is 25mg or 50mg, taken 3 or 4 times a day. Try to space your doses evenly throughout the day.
• Insect bites, stings and eczema - diphenhydramine comes as a cream which you use once or twice a day. You'll only need a small amount, enough for a thin layer.

How to use it

You can take diphenhydramine tablets, capsules and liquid with or without food.

Always take your diphenhydramine tablets or capsules with a drink of water. Swallow them whole. Do not chew them.

Liquid medicines containing diphenhydramine come with a plastic syringe or spoon to help you measure out the right dose. If you don't have a syringe or spoon, ask your pharmacist for one. Do not use a kitchen teaspoon as you won't get the right amount.

With the cream, put a small amount onto the affected area and smooth it in gently. Do not use it on large areas of skin. Keep the cream away from your eyes, mouth and nose. Remember to wash your hands afterwards - unless you're treating your hands.

What if I forget my medicine?

If you're taking diphenhydramine to help you sleep and you forget to take it by bedtime, skip your missed dose and start again the next evening.

If you're taking any other medicine containing diphenhydramine and forget to take a dose, take your medicine when you next need it to ease your symptoms.

Never take 2 doses at the same time. Never take an extra dose to make up for a forgotten one.

If you forget doses often, it may help to set an alarm to remind you. You could also ask your pharmacist for advice on other ways to help you remember to take your medicine.

What if I use too much?

Too much diphenhydramine can be dangerous.

If you've taken too much by accident:

• you may feel very sleepy or sick
• your eyesight may be blurry
• your heart may beat very fast

In serious cases you can become unconscious and may need emergency treatment in hospital.

Do not use diphenhydramine cream at the same time as other products containing diphenhydramine. This can lead to overdose.

Like all medicines, diphenhydramine can cause side effects, although not everyone gets them. However, you may be more likely to get them if you're over 65.

Common side effects

Common side effects happen in more than 1 in 100 people. Talk to your doctor or pharmacist if these side effects bother you or don't go away:

• feeling sleepy during the daytime
• dry mouth
• feeling dizzy or unsteady on your feet, or having difficulty concentrating

Diphenhydramine cream can sometimes make rashes worse. It may also make your skin sensitive to sunlight.

Serious side effects

Tell your doctor straight away if you:

• have an irregular heartbeat
• an increased awareness of your heartbeat
• feel any numbness or pins and needles
• feel confused or very restless
• have a fit (seizure)

If someone around you is having a fit, call 999 straight away.

Serious allergic reaction

In rare cases, it's possible to have a serious allergic reaction (anaphylaxis) to diphenhydramine.

These are not all the side effects of diphenhydramine. For a full list, see the leaflet inside your medicine packet.

What to do about:

• feeling sleepy during the daytime - drowsiness usually wears off 8 hours after a dose. Do not drive or use tools or machinery if you're feeling this way.
• dry mouth - chew sugar-free gum or suck sugar-free sweets.
• feeling dizzy or unsteady on your feet, or difficulty concentrating - if diphenhydramine makes you feel this way, stop what you're doing and sit or lie down until you feel better. If the feeling doesn't go away or is troubling you, do not take any more medicine and speak to a pharmacist or your doctor.

Diphenhydramine isn't normally recommended during pregnancy.

This is because there isn't enough information to be sure it's safe. If you take diphenhydramine towards the end of your pregnancy, there's a risk your baby may be born with some of the side effects.

If you have insomnia while you're pregnant, your doctor or midwife may suggest ways to improve your sleep routine - like relaxing, and avoiding naps. If this doesn't work, your doctor may prescribe a different antihistamine called promethazine to help you sleep.

If you have a cough or cold, you can help to ease your symptoms by resting, drinking plenty of fluids and taking everyday painkillers such as paracetamol.

Diphenhydramine and breastfeeding

Diphenhydramine isn't normally recommended when breastfeeding, as small amounts of the medicine pass into your breast milk. It may also reduce the amount of milk you produce.

Talk to your doctor, as other medicines might be better while you're breastfeeding. It's usually safe to take similar antihistamines called loratadine and cetirizine while you're breastfeeding. If you need a drowsy antihistamine to help you sleep, your doctor may recommend chlorphenamine.

However, speak to your doctor before taking any antihistamine if your baby was premature, had a low birth weight or has health problems.

Some medicines and diphenhydramine interfere with each other and increase the chances of you having side effects. Check with your pharmacist or doctor if you're taking:

• antidepressants, such as venlafaxine or a monoamine oxidase inhibitor such as phenelzine
• metoprolol, a heart medicine
• any medicine that makes you drowsy, gives you a dry mouth, or makes it difficult for you to pee - taking diphenhydramine might make these side effects worse

If you're taking a cough or cold remedy or a painkiller containing diphenhydramine, check carefully what the other ingredients are. For example, check whether it contains paracetamol. Ask your pharmacist for advice before you take this medicine together with any other painkillers or medicines.

Mixing diphenhydramine with herbal remedies and supplements

There might be a problem taking some herbal remedies and supplements alongside diphenhydramine - especially ones that cause side effects such as sleepiness, a dry mouth or make it difficult to pee.