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.
How do they 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?
Generally, topical antihistamines do not usually cause side effects, although occasionally there may be local irritation. With oral antihistamines, the older, sedating types may cause difficulty in passing urine and therefore may not be suitable for men with prostate problems. These drugs also may not be suitable for some people with glaucoma. If you have either of these conditions, you should check with your pharmacist or doctor before taking an older type of antihistamine.
Oral antihistamines may also cause varying degrees of drowsiness, problems with coordination, dry mouth, constipation, and blurred vision. These problems are more likely with sedating antihistamines but may also occur with the nonsedating ones. If an antihistamine causes drowsiness, coordination problems, and/or blurred vision, you should not drink alcohol, drive, or operate machinery. In some children, antihistamines may cause hyperactive behaviour.
Oral antihistamines may interact with a variety of other medications, including some antidepressants, beta-blockers, and antibacterial drugs. If you are taking any medications, you should check with your pharmacist or doctor before taking an antihistamine.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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