Diphenhydramine

What is diphenhydramine?

Diphenhydramine is a sedating antihistamine. Most commonly, it’s used to relieve allergy and cold symptoms. However, it can also help with short-term sleep problems such as insomnia. You may find it available under brand names such as Nytol, Sleepeaze, and Histergan[1]. 

What is diphenhydramine used for?

Diphenhydramine is used to treat a range of allergy symptoms including hay fever, hives, eczema, and insect bites. It can also ease short-term sleep problems, including when cold symptoms, such as sneezing and a runny nose, or itching is keeping you awake[1]. 

How does diphenhydramine work?

Diphenhydramine works by blocking the action of histamine, a chemical in the body that causes allergic reactions. It also has sedative properties. Combined, this can ease irritating symptoms while making you feel sleepy. This is why it is sometimes used as a sleep aid[1]. 

Who can take diphenhydramine?

Diphenhydramine is generally safe for adults and children aged 16 years and over. You can also sometimes give diphenhydramine to children, depending on age and symptoms[1].  

However, it may not be suitable for people who have glaucoma, a stomach blockage, issues peeing, asthma, an enlarged prostate, myasthenia gravis, liver, lung, or kidney problems, or epilepsy. You should not take it if you can’t drink any alcohol, are due an allergy test or have had an allergic reaction to it before[1]. 

Please tell your doctor or pharmacist about any conditions you may have before you start treatment and always follow the instructions in the Patient Information Leaflet[2]. 

How do you take diphenhydramine?

Diphenhydramine is available in different forms. You can find it available as tablets, capsules, liquids, creams, and injections (used in hospitals). Usually, you should swallow tablets or capsules whole with water, or use a proper measuring spoon for liquid forms. They can be taken without food.  

Liquid diphenhydramine should come with a plastic syringe or spoon. Do not use a kitchen teaspoon. The cream form should be applied to the affected area and smoothed in gently. 

Always follow the dosage instructions on the packaging or given by your doctor no matter which form of diphenhydramine you receive[1]. 

What is the typical dosage of diphenhydramine?

Dosage depends on the condition being treated[1]: 

• Short-term insomnia – The usual dose is 50mg, 20 minutes before bedtime 

• Hay fever & allergies - The usual dose is 25mg or 50mg taken 3 or 4 times evenly spread through the day 

• Insect bites & stings – Apply the cream on the affected area once or twice a day 

When using diphenhydramine for motion sickness you will be given a dose to take in the hours before travelling. Always follow the advice on the label or from your doctor. 

Can you stop taking diphenhydramine suddenly?

It’s usually safe to stop taking diphenhydramine when you no longer need it. However, if you have been using it regularly for sleep, speak to a doctor before stopping, especially if you have trouble sleeping without it. In this case you may need to reduce doses over time[1]. 

What should you do if you take too much diphenhydramine?

Taking too much diphenhydramine can produce symptoms like severe drowsiness, a fast or irregular heartbeat, seizures, and hallucinations. If you or someone else may have taken too much diphenhydramine, call 999 or go to A&E[1]. 

What should you do if you forget a dose of diphenhydramine?

If you forgot to take a dose of diphenhydramine, the best course of action depends on how long it is until your next dose. You can take the missed dose as usual if only a short time has passed since it was due. Alternatively, if your next dose is due soon, just skip the missed dose altogether and carry on your schedule. Do not take a double dose to make up for missing one[1]. 

What medicines should you avoid while taking diphenhydramine?

You should avoid taking diphenhydramine with other sedating antihistamines, sleeping tablets, antidepressants, anxiety medication, metoprolol (heart medicine), any drowsy medication, and muscle relaxants as these can have severe side effects. It’s also best to avoid drinking alcohol when taking diphenhydramine[1]. 

Always inform your doctor or pharmacist about all medications and supplements you take before you start a new medicine, and follow the instructions in the Patient Information Leaflet[2]. 

What are the side effects of diphenhydramine?

The usual side effects when taking diphenhydramine may include drowsiness, a dry mouth, dizziness, skin sensitivity to sunlight,eczema or a skin rash. Rare, but more serious side effects include experiencing confusion, especially in older adults, having difficulty urinating, restlessness, numbness, or a fast heartbeat[1]. 

You should contact your GP if you experience any of these symptoms. In rare cases, the medicine may cause a serious allergic reaction (anaphylaxis). You should call 999 or visit A&E immediately if you believe this is happening[1]. 

If you experience any side effects, including those not listed, you are encouraged to report them via the Yellow Card Scheme or contact your doctor, pharmacist, or nurse. This information is intended as a summary for general awareness and does not replace professional medical advice. 

The information provided in the side effects section is not a comprehensive list of all possible side effects associated with this medication. For a complete list of side effects, please refer to the Patient Information Leaflet (PIL) provided with your medication or consult your healthcare professional[2]. 

Frequently asked questions about diphenhydramine

Can I drive or operate machinery while taking diphenhydramine? 

No, diphenhydramine can cause drowsiness and impair your ability to concentrate. You should not drive or use machinery after taking it[1]. 

Can I drink alcohol while taking diphenhydramine? 

Alcohol increases the sedative effects of diphenhydramine and can make you extremely drowsy, so you should avoid it while taking the medicine[1]. 

Is diphenhydramine safe during pregnancy or breastfeeding? 

Taking diphenhydramine while pregnant or breastfeeding is not usually recommended. Speak to your doctor for advice[1]. 

Can I take diphenhydramine long-term? 

Diphenhydramine is intended for short-term use in most cases. Long-term use, particularly for sleep, is generally not advised without medical supervision. Speak to your doctor if you have any concerns about your diphenhydramine prescription[1]. 

For the official NHS guide to diphenhydramine, visit their website.