Antihistamines are drugs that are used to treat a number of allergic health conditions by blocking the effects of a protein called histamine.
Antihistamines are available in tablet or capsule (oral) form, as creams, lotions and gels (topical form), and as a nasal spray.
Antihistamines work by preventing the actions of histamine, which is a protein produced by the body as part of its natural defences. Although histamine is generally useful, it is sometimes necessary to use antihistamines to block its effects. For example, when there is an excessive allergic reaction to harmless substances such as pollen.
Serious drowsiness can be caused by so-called 'first-generation' antihistamines such as diphenhydramine and chlorphenamine. People who regularly take first-generation antihistamines are more likely to be involved in serious accidents. Second-generation antihistamines in general do not cause drowsiness; examples of these include loratadine and cetirizine.
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