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Antihistamine drugs

Antihistamine drugs are used to stop the effects of a substance in the blood called histamine. Histamine is a natural chemical produced by the immune (disease-fighting) system of the body. When it is released, it increases blood flow to the affected area. Liquid leaks out of the blood vessels, making the mucous membranes lining the nose and throat to swell, and stimulating nearby gland to produce mucus. It causes symptoms such as itching, swelling, or reddening. Histamine can also cause sneezing, running nose, itchy or watery eyes, and in some people can cause breathing difficulties.

The main action of histamine is related to allergies. It is released when the body comes in contact with an allergen. An allergen is a substance that isn’t normally harmful, such as pollen, but which causes an over-reaction by the immune system in some people. Histamine also has other functions around the body, such as widening and tightening blood vessels and squeezing muscles in your breathing and digestive systems.

Antihistamine drugs prevent histamine carrying out these actions. They are most often used to treat allergic reactions, although certain types can be used in the treatment of other conditions such as migraines, travel sickness, and stomach ulcers.

Antihistamine drugs are available as tablets (oral), creams, and injections. The older antihistamines tend to cause drowsiness (sedation is the medical term for drowsiness) and rare dangerous heart arrhythmias (terfenadine and astemizole). More recently developed, new antihistamines, cause less drowsiness. The effects of ‘new’, non-sedating antihistamines also tend to last longer, so you have to take the medicine less frequently.

Some antihistamines, such as creams for insect bites, and some tablets for hay fever, can be bought over-the-counter from your pharmacist. Others have to be prescribed by your doctor.