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Published by Bupa's health information team, September 2009.
This factsheet is for people who are taking antihistamine medicines or who would like information about taking them.
Antihistamines are most commonly used to treat allergies such as hay fever. They are also ingredients in other types of medicines, including treatments for migraine and travel sickness, and sleep aids.
Why would I take antihistamines?
You might take an antihistamine if you have symptoms of allergies, such as:
- hay fever
- allergic rashes
- itchy skin
Certain antihistamines block the histamine receptors in the brain. These can be useful as a treatment for travel sickness, vertigo or sickness related to migraine. They can also help to relieve tickly coughs.
Some antihistamines cause sleepiness, and there are some types you can take to help you sleep (eg over-the-counter sleep aids, such as Nytol, which contain the antihistamine diphenydramine that causes drowsiness).
What are the main types of antihistamines?
There are two main types of antihistamines - sedating and non-sedating.
Sedating antihistamines work in your brain as well as the rest of your body, and they can cause sleepiness. Examples include promethazine and chlorphenamine.
Non-sedating antihistamines don't pass into your brain so easily, so they don't make you sleepy. This also means that they don't relieve sickness. Examples include cetirizine and desloratadine.
How do antihistamines work?
When an unknown and potentially harmful substance enters your body, your immune system reacts by launching various defences to protect you (your immune response).
Histamine is one of the chemicals released during an immune response. Its job is to help damaged tissue to heal, and in doing this, it causes swelling, itching and redness.
If you're allergic to something, your body mistakenly mounts an immune response to something that's not really harmful, and releases histamine. For example, with hay fever your body mistakes pollen for a harmful substance. You then get symptoms such as sneezing and red, itchy eyes. For more information on hay fever, see Related topics.
Antihistamines work by blocking the action of histamine, and so easing the symptoms of allergic reactions.
How to take antihistamines
Many products based on antihistamines are available without prescription from a pharmacy. Examples include chlorphenamine (eg Piriton), loratadine and cetirizine. These are usually only for short-term use. Higher doses can only be prescribed by your doctor.
Antihistamines come as tablets, capsules and syrups (oral preparations), eye drops, nasal sprays and drops, creams and injections.
If you have hay fever, there are several different antihistamine preparations available to treat the symptoms. It may be best to start these before the pollen season starts, so ask your pharmacist or GP for advice.
Antihistamine creams can cause allergic-type reactions themselves, and you shouldn't use them for more than three days. The risks might outweigh the benefits, so ask your pharmacist for advice.
Antihistamine injections are only available on prescription. They are only used for severe allergic reactions and anaphylactic shock (a potentially life-threatening allergic reaction).
Some antihistamines may be unsuitable for children under 12. Check the label on the medicine or ask your pharmacist for advice. Always read the patient information leaflet that comes with your medicine.
Talk to your GP or pharmacist before taking an antihistamine if you have a medical condition or are taking any other medicines.
You should also tell your GP if you are pregnant or breastfeeding before taking an antihistamine.
Side-effects of antihistamines
The list of side-effects below does not include every possible side-effect of antihistamines. Read the patient information leaflet that comes with your medicine for more information.
Sedating antihistamines might make you feel sleepy, though this may improve after you have been taking the medicine for a few days. Non-sedating antihistamines rarely cause drowsiness. Drowsiness caused by antihistamines can make it unsafe to drive or operate machinery.
Other, less common side-effects, mainly from sedating antihistamines, include:
- difficulty in passing urine
- dry mouth
- blurred vision
- feeling sick or vomiting
- constipation or diarrhoea
- skin or eye irritation (from creams, lotions and eye drops)
The following side-effects have also occasionally been reported:
- palpitations and abnormal heart rhythms (arrhythmia)
- high blood pressure
- allergic reactions (such as swelling, rash and breathing difficulties)
- disturbed sleep
- blood and liver disorders
- over-excitement in children
Children and adults over 65 are more likely to get side-effects.
Interactions with other medicines
Check with your GP or pharmacist before you take any other medicines or herbal remedies at the same time as an antihistamine.
The antihistamine mizolastine can interact with some other medicines to cause a serious abnormal heart rhythm. This medicine is only available on prescription from your doctor, who will give you advice if you need to take it.
Be aware that alcohol can increase the sedating effects of antihistamines.
Names of common antihistamines
The main types of antihistamine are shown in the table.
All medicines have a generic name. Many medicines also have one or more brand name. Generic names are written in lower case and brand names start with a capital letter.
||Brand names (some of these products may also contain other medicines)
||Benadryl Allergy Relief, Benadryl Plus Capsules
||Benadryl Allergy Oral Syrup, Benadryl for Children Allergy Solution, Benadryl One A Day Relief, Piriteze Allergy, Pollenshield Hayfever Relief, Zirtek Allergy Relief
||Allercalm Allergy Relief Tablets, Piriton
|For the eyes and nose
||Otrivine-Antistin (for eyes)
||Optilast (for eyes), Rhinoblast (for nose)
||Emadine (for eyes)
||Relestat (for eyes)
|Creams and lotions for the skin
||Benadryl Skin Allergy Relief Cream
||Anthisan, Wasp-Eze Bites and Stings spray, Wasp-Eze
|Travel sickness, vertigo/nausea and sleep aids
||Nytol, Nytol One-a-Night
See our answers to common questions about antihistamine, including:
electronic Medicines Compendium
- Joint Formulary Committee. British National Formulary. 57th ed. London: British Medical Association and the Royal Pharmaceutical Society of Great Britain, 2009
- Simon C, Everitt H, Kendrick T. Oxford handbook of general practice. 2nd ed. Oxford: Oxford University Press, 2006: 920
- Medicine chest online. Proprietary Association of Great Britain. www.medicinechestonline.com, accessed 28 May 2009
This information was published by Bupa's health information team and is based on reputable sources of medical evidence. It has been peer reviewed by Bupa doctors. The content is intended for general information only and does not replace the need for personal advice from a qualified health professional.
Publication date: September 2009