Diphenhydramine: the drowsy antihistamine for allergies and short-term sleep problems

Antihistamines block the effects of histamine, the chemical the body releases when it comes into contact with something it is allergic to. The NHS explains that diphenhydramine blocks histamine in the brain, which reduces symptoms but also causes drowsiness, because it enters the brain in large quantities. It also blocks another chemical, acetylcholine, which can help a cough or runny nose but causes a dry mouth and dry nose.

Because it makes people sleepy, diphenhydramine is classed as a sedating antihistamine. The NHS contrasts it with non-drowsy antihistamines such as cetirizine, fexofenadine and loratadine, which are less likely to affect everyday routines. Other drowsy antihistamines include chlorphenamine, promethazine and hydroxyzine.

In the United States the same ingredient is sold as Benadryl; in the UK the NHS lists brand names such as Nytol Original, Boots Sleepeaze and Histergan. It can be bought from pharmacies and is also available on prescription, as tablets, a liquid or a cream.

The NHS lists hay fever, eczema, hives (urticaria), insect bites and stings, and short-term sleep problems (insomnia), including when a cough, cold or itching is keeping you awake at night. For skin allergies like hives or bites the cream is much less likely to make you sleepy than the tablets or liquid.

Diphenhydramine is also included in some cough and cold remedies mixed with levomenthol, paracetamol, pseudoephedrine, ammonium chloride and dextromethorphan. The NHS names Benylin Chesty Coughs, Benylin Children’s Night Coughs, Bell’s Dual Action Chesty Cough, Covonia Night Time Formula, Numark Chesty Cough Expectorant and Panadol Night Pain as examples. Because the same ingredient can appear in several products, read the active ingredients and avoid taking two at once.

The NHS says that for cough and cold symptoms, symptoms normally start to improve within 20 minutes. It adds that some cough and cold remedies claim to suppress cough or stop you bringing up phlegm, and that although some people find them helpful they are not usually recommended because there is little evidence they are more effective than simple home remedies and they are not suitable for everyone.

For sleep, you will usually take it 20 minutes before bed and it normally takes about 30 minutes to work, according to the NHS. The effect lasts about 4 to 6 hours and drowsiness usually wears off around 8 hours after a dose.

The NHS says most adults and young people aged 16 and above can take diphenhydramine. For cough and cold symptoms, tablets and liquid containing diphenhydramine can be given to children aged 6 years and over, and for allergy symptoms to children aged 12 and over. Cream can be used on children aged 6 and over for insect bites, stings and eczema. It warns that diphenhydramine should not be given to a child to help them sleep, and is only suitable for sleep problems from 16 years.

Tell your doctor or pharmacist if you have had an allergic reaction to it, have lung problems such as asthma or COPD, or an illness that creates a lot of phlegm, primary angle closure glaucoma, a blockage in your stomach or gut, a stomach ulcer, kidney or liver problems, problems peeing or emptying your bladder, epilepsy or seizures, myasthenia gravis, or confusion if you are older. Allergy tests can be affected, so you may need to stop for a few days before one. Some liquid products contain a small amount of alcohol.

The NHS notes that side effects are more likely if you are over 65.

Common side effects, affecting more than 1 in 100 people, include feeling sleepy or drowsy in the daytime, dry mouth, and feeling dizzy, unsteady or having difficulty concentrating. The NHS suggests that if it makes you feel dizzy or unsteady you should stop what you are doing and sit or lie down, and not drive, ride a bike or use machinery. The cream can make skin more sensitive to sunlight, so use a high factor sunscreen of at least SPF 30. If you get a new rash or eczema where you have used the cream, stop and speak to a doctor.

Call a doctor or NHS 111 straight away if you have a fast or irregular heartbeat, numbness or pins and needles, feel very restless or feel confused. Call 999 if you or your child has had a seizure. A serious allergic reaction is rare but needs 999: swelling of the lips, mouth, throat or tongue, very fast or difficult breathing, a tight throat, blue, grey or pale skin, sudden confusion or dizziness, or fainting.

Report suspected side effects through the Yellow Card scheme.

The NHS says not to drink alcohol while taking diphenhydramine, because together they can make you sleep very deeply, you may not be able to breathe properly and may have difficulty waking up. If you take it for sleep, avoid caffeine from coffee, tea, cola or energy drinks, because caffeine has the opposite effect and can stop it working.

On driving, do not drive or ride a bike if it makes you sleepy during the day, gives blurred vision or makes you feel dizzy, clumsy or unable to concentrate. It is an offence to drive if your ability to drive safely is affected, and it is your responsibility to decide whether it is safe.

Do not take another drowsy antihistamine together with diphenhydramine, as it will make you very tired. The NHS says that for a severe itchy rash, a doctor may recommend a non-drowsy antihistamine by day and a drowsy one at night, but not two together unless advised. Diphenhydramine does not affect contraception, including the combined pill and emergency contraception, according to the NHS.

It is unlikely that you will become addicted, but the NHS says you can become dependent if you take it without a break for too long, and it may stop working as well. Only take it for a very short time unless your doctor advises otherwise. Speak to your doctor if you need it for more than 2 weeks.

The NHS suggests lifestyle steps for sleep: regular bed and waking times, relaxing before bed, using thick curtains or an eye mask and earplugs, avoiding caffeine, nicotine, alcohol, heavy meals and exercise for a few hours before bed, avoiding screens in bed and daytime naps, and writing down worries before going to bed. For hay fever, the NHS suggests staying indoors when the pollen count is high, wearing wraparound sunglasses, putting petroleum jelly around the nostrils and showering after being outside.

Diphenhydramine is mostly bought from pharmacies without a prescription, and pharmacists can advise on suitability. If sleep problems continue for more than a few weeks, a GP can look for causes and discuss non-drug approaches. The same applies whether you see an NHS or private clinician.