What are antihistamines? - Allergy treatments

Antihistamines, or histamine antagonists, are a type of drug that act on H1 receptors. H1 receptors are found on various tissues and respond to histamine. Histamine is released by mast cells (a type of immune cell) when they are triggered by IgE, which responds to an allergen. These are extremely common and you can buy them over the counter in pharmacies and supermarkets. They are effective against a wide range of symptoms as well.

There are two types of antihistamines. The first are known as first generation antihistamines. These are older and cheaper than newer formulations, however, they can also cross the blood-brain barrier and bind to receptors in the central nervous system as well. Common side effects of first generation antihistamines include drowsiness and sleepiness. Drugs such as alimemazine or promethazine may cause more sleepiness, whereas drugs such as chlorphenamine and cyclizine may cause a little less drowsiness. Drowsiness may affect tasks such as driving, and drinking alcohol will make you more sleepy. Other common side effects included headaches, a dry mouth, urine retention, or blurred vision. Nowadays, first generation antihistamines are often used as part of drugs to help people sleep. There are a few different classes of first generation histamines, these classes are based on the chemical structure of the drug. Examples of first generation antihistamines include alimemazine, promethazine, cyclizine, chlorphenamine, or ketotifen.

Second generation antihistamines cross the blood-brain barrier to a much lesser extent, as a result, drowsiness is much rarer (but still can happen). As is the case with first generation antihistamines, any drowsiness can be made worse by alcohol. Common second generation antihistamines include loratadine, cetirizine hydrochloride, or desloratadine.

Less common side effects of antihistamines include a low blood pressure, abnormalities in your heart rhythms, palpitations (being aware of your heart beating), confusion, dizziness, mood changes, liver problems, rashes, or sleep disturbances. Some antihistamines (such as loratadine) should be avoided in pregnancy or breast-feeding, and care should be taken if you have liver or kidney problems, or epilepsy. Children and older people are more susceptible to the side effects caused by antihistamines.

Antihistamines exist in a variety of forms. Tablets are commonly found. Nasal sprays target the nose specifically, and can be used in allergic rhinitis (such as that cause by hay fever, mould allergies, pet allergies, or house dust mite allergies). Eye drops also exist for symptoms of allergic conjunctivitis.

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