Common questions about diphenhydramine - Brand names: Nytol Original, Boots Sleepeaze, Histergan

Diphenhydramine is a type of medicine called an antihistamine. It's classed as a drowsy (sedating) antihistamine.

When you come into contact with something you're allergic to, your body produces a chemical called histamine. This can cause symptoms such as a runny nose or skin rashes.

Diphenhydramine blocks the effects of histamine in your brain and this reduces symptoms. It enters the brain in large quantities and this can make you feel drowsy.

Diphenhydramine also blocks the effects of another chemical called acetylcholine. This can help a cough or runny nose, but can also cause side effects such as a dry mouth and dry nose.

If you're taking diphenhydramine for short-term sleep problems, it will start to make you feel drowsy around 20 to 30 minutes after you take it.

For coughs and colds, your symptoms will normally start to improve within 20 minutes.

The medicine should work for around 4 to 6 hours.

It's unlikely that you'll become addicted to diphenhydramine, but it's best to only take it for a short time. You can become dependent on diphenhydramine if you take it without a break for too long.

Follow the instructions that come with the medicine and only take diphenhydramine for a very short time, unless your doctor gives you different advice.

If you have a severe itchy skin rash, your doctor may recommend taking 2 different antihistamines at the same time for a few days.

As well as taking a non-drowsy antihistamine during the day (such as fexofenadine, cetirizine or loratadine), your doctor may advise that you take a drowsy antihistamine at night if itching is making it difficult to sleep.

Do not take another drowsy antihistamine together with diphenhydramine. It will make you very tired and sleepy. Other drowsy antihistamines include chlorphenamine (Piriton), promethazine (Phenergan) and hydroxyzine (Atarax or Ucerax).

Diphenhydramine is known as a drowsy, or sedating, antihistamine as it makes you sleepy.

Non-drowsy antihistamines are less likely to have this effect. These include cetirizine, fexofenadine and loratadine.

Most people prefer to take a non-drowsy antihistamine as it's less likely to interfere with their everyday routine. But you might choose to take a drowsy antihistamine if you're having problems falling asleep, or if symptoms like itching or coughing are keeping you awake.

There is not enough information to say whether one antihistamine is better than another for treating allergies. You may need to try a few different medicines to find what works best for you. Ask your pharmacist or doctor for advice if you're unsure.

Diphenhydramine blocks the effects of a natural chemical called acetylcholine. This can help dry up a cough or runny nose but can also cause side effects such as a dry mouth and dry nose.

Some cough and cold remedies claim to suppress your cough or stop you bringing up phlegm. Although some people find them helpful, they're not usually recommended. This is because there's little evidence that they're any more effective than simple home remedies, and they're not suitable for everyone.

Diphenhydramine does not affect any type of contraception including the combined pill and emergency contraception.

Do not drive a car or ride a bike if diphenhydramine makes you sleepy during the day, gives you blurred vision or makes you feel dizzy, clumsy or unable to concentrate or make decisions.

This may be more likely when you first start taking diphenhydramine but could happen at any time, for example when starting another medicine.

It's an offence to drive a car if your ability to drive safely is affected. It's your responsibility to decide if it's safe to drive. If you're in any doubt, do not drive.

Talk to your doctor or pharmacist if you're unsure whether it's safe for you to drive while taking diphenhydramine. GOV.UK has more information on the law on drugs and driving.

Do not drink alcohol while you're taking diphenhydramine.

Alcohol and diphenhydramine together can make you sleep very deeply. You will not be able to breathe properly, and you may have difficulty waking up.

You can eat normally while taking diphenhydramine.

However, do not drink alcohol.

If you're taking diphenhydramine to help you sleep, try not to drink anything that contains caffeine, such as coffee, tea, cola or energy drinks. Caffeine has the opposite effect to diphenhydramine on your body and can stop it working.

There are a number of things you can try to help improve sleep and tiredness. You could try:

• setting regular times for going to bed and waking up
• relaxing before bedtime – try taking a warm bath or listening to calming music
• using thick curtains or blinds, an eye mask and earplugs to stop you being woken up by light and noise
• avoiding caffeine, cigarettes or vapes, alcohol, heavy meals and exercise for a few hours before bedtime
• not watching TV and not using phones, tablets or computers in bed, or before bedtime
• avoiding naps during the day
• making a list of your worries and any ideas for how to solve them before you go to bed – this can help you forget about them until the morning

If you have hay fever, it will help if you do not spend too much time outside when the pollen count is high.

Tips for when you're outside

• Do not cut grass or walk on grass.
• Wear wraparound sunglasses to stop pollen getting into your eyes.
• Put Vaseline (petroleum jelly) around your nostrils to help trap pollen.
• Shower and change your clothes after you've been outside to wash off pollen.

Tips for when you're inside

• Keep windows and doors shut as much as possible.
• Vacuum regularly and dust with a damp cloth.
• Do not keep fresh flowers in the house.
• Do not smoke or be around smoke because it makes hay fever symptoms worse.

The best way to prevent any sort of allergic reaction is to avoid the substance that you're allergic to, if you can.