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AntihistaminesThis factsheet is for people who would like information about taking antihistamines. Antihistamines are most commonly used to treat allergies such as hay fever. They are also ingredients in other types of medicines, including cough and cold remedies, travel sickness treatments and sleep aids.
Why would I take them?You might take an antihistamine if you have symptoms of allergies, such as:
Certain antihistamines block the histamine receptors in the brain. These can be useful as a treatment for travel sickness or vertigo and 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). How do antihistamines work?When a germ gets inside your body, your immune system recognises it as a foreign substance and triggers various defences to protect you. The body's defences are called an immune response, and include making chemicals that kill off germs. When the cells that make up your body's tissues are damaged, they release a chemical called histamine. Histamine acts like a messenger to nearby cells, telling them to start up their own defences. This makes the site where the germ entered the body (the site of infection) red, swollen and sore. If you are allergic, your body mistakenly mounts an immune response to something that's not really harmful. For example, with hay fever your body mistakes pollen for a harmful substance and releases histamine. You then get symptoms such as sneezing and red, itchy eyes. For more information, please see the separate BUPA health factsheet, Hay fever. Antihistamines work by blocking the action of histamine on other cells, and so easing the symptoms of allergic reactions. How to take antihistaminesMany products based on antihistamines are available without prescription from a pharmacy. Examples include chlorphenamine (eg Piriton) and desloratidine (eg Neoclarityn). 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 lotions, and injections. The nasal sprays can be helpful for hay fever and it's best to start using these before your symptoms appear. You should not take non-prescription nasal steroids for hay fever for more than three months. If your symptoms continue, see your GP. The eye drops are useful if only your eyes are affected by hay fever or if, even after taking tablets, your eyes still itch. The creams and lotions can be helpful but can cause allergic-type reactions themselves. The risks might outweigh the benefits, so ask your pharmacist for advice. Antihistamine injections are only available on prescription and are only used for severe allergic reactions and anaphylactic shock (a potentially life-threatening allergic reaction). Antihistamines and childrenSome antihistamines may be unsuitable for children under 12. Check the label or ask your pharmacist for advice. Always read the patient information leaflet that comes with your medicine. Special careTalk to your doctor or pharmacist before taking an antihistamine if you have a medical condition or are taking any other medicines. You should also tell your doctor if you are pregnant or breastfeeding before taking an antihistamine. Side-effectsThere are two types of antihistamines. The older (sedating) ones such as chlorphenamine are more likely to make you feel sleepy. The newer (non-sedating) ones such as loratadine rarely cause drowsiness. Drowsiness caused by antihistamines can make it unsafe to drive or operate machinery. Other, less common side-effects, mainly from the sedating group of antihistamines, are:
Rarely, some antihistamines can also cause:
Children, and adults over 65, are more likely to get side-effects. Interactions with other medicinesCheck with your doctor or pharmacist before you take any other medicines or herbal remedies at the same time as an antihistamine. The antihistamines terfenadine and mizolastine can interact with some other medicines and with grapefruit juice to cause a serious abnormal heart rhythm. These antihistamines are only available on prescription from your doctor, who will give you advice. Be aware that alcohol can increase the sedating effects of antihistamines. Names of common antihistaminesExamples of the main types of antihistamines are shown in the table. The generic name is the chemical name of a medicine. If a manufacturer has rights over a medicinal compound, it is given a brand name. Often there is more than one brand name associated with a generic name. Generic names are normally written with a lower-case initial letter and brand names normally start with an upper-case letter.
Further information
Sources
Published by BUPA's health information team, healthinfo@bupa.com, February 2007.
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