Antihistamines

Antihistamines are drugs that block the effects of histamine, a chemical released during allergic reactions.

Antihistamines are used mainly to prevent or relieve symptoms of allergies, such as hay fever (see Allergic rhinitis), and to treat allergic rashes, such as urticaria. These drugs are also effective in relieving itching and irritation due to insect bites or stings. Antihistamines are sometimes included in cold and flu remedies because they dry up mucus. Since some types of antihistamine also depress the vomiting reflex in the brain, these drugs may also be taken to relieve nausea, vomiting, vertigo, and motion sickness (see Antiemetic drugs). Certain antihistamines have a sedative effect and are recommended if you have regular night-time itching.

Antihistamines are normally taken orally, but some types are available for topical use as nasal sprays, eyedrops, or skin lotions. Antihistamines may also be given by injection as emergency treatment for the life-threatening allergic reaction anaphylaxis.

Common drugs

Sedating antihistamines: Alimemazine (trimeprazine), Brompheniramine, Chlorphenamine (chlorpheniramine), Clemastine, Cyproheptadine, Diphenhydramine, Promethazine

Nonsedating antihistamines: Acrivastine, Cetirizine, Loratadine

How do antihistamines work?

Antihistamines prevent or relieve the symptoms of an allergic reaction by blocking the action of histamine, a chemical that is released from certain cells in the blood and body tissues when they are exposed to an allergen (see How antihistamines work). The effects of histamine include widening of the small blood vessels in the affected area, often the skin, nose, and eyes, leading to redness and swelling and increased production of mucus. By blocking the action of histamine, antihistamine drugs can reduce some of the symptoms of allergies, such as swelling, rash, itching, runny nose, and sneezing.

What are the side effects of antihistamines?

Many antihistamines make you drowsy and cause problems with coordination. If you are affected, you should not drive or operate machinery. However, in some children, antihistamine drugs may cause hyperactive behaviour.

All types of antihistamine can cause difficulty in passing urine, and the drugs should not be taken by men who have prostate problems. You should not drink alcohol while you are taking antihistamines because this will increase the sedative effect of the drugs.

BMA Complete Family Health Guide Copyright © 2005 Dorling Kindersley

Posted 09.09.2010

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