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  • Subject area(s): Science
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  • Published on: 15th October 2019
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Antihistamines are a class of medication used to treat the symptoms of allergies and reactions, and are available on both prescription and over the counter. (NHS, 2017) Over the counter antihistamines are one of the most popular lines of treatment for allergies, colds and coughs. (Church et al., 2010)  With 56% of under 14’s receiving antihistamine treatment, the safe and effective prescribing of these drugs must be ensured. (Anagnostou, Swan and Brough, 2016)

The chemistry of antihistamines affects their activity on the body. Antihistamines work by binding to cell surface histamine receptors, of which there are four types, and as a result stabilising the receptor whilst inactive. (Randall and Hawkins, 2018) In general, antihistamines are liposoluble, causing them to have good bioavailability due to them easily crossing cell membranes. (del Cuvillo et al., 2006) In the past, antihistamines were long considered to be agonists, however it is now accepted that they are in fact inverse agonists. (del Cuvillo et al., 2006)  Inverse agonists work by binding to a specific receptor and as a result reducing its function. (Khilnani and Khilnani, 2011) In general, most antihistamines are metabolised by cytochrome P450, with a few second-generation drugs not being metabolised in the liver. (Fitzsimons et al., 2014) There are some significant differences between first and second-generation antihistamines. Absorption of oral antihistamines normally takes place between one and three hours, however some second-generation drugs absorb faster, for example cetirizine and levocetirizine. (Fitzsimons et al., 2014) Second-generation antihistamines have a better profile, with a longer half life and a 24 hour duration of action. Allegaert and Choonara, 2016) Antihistamines have a wide range of interactions, such as their sedative effect being exacerbated by hypnotics and alcohol, and their antimuscarinic effect being heightened by certain antidepressants. (Fitzsimons et al., 2014)  For young children, it is recommended that their dose be split up, due to how fast the drugs are eliminated. (Fitzsimons et al., 2014)

Clinically antihistamines can be used to treat a wide variety of conditions in children, including allergies, conjunctivitis and hay fever. (NHS, 2017) There are some conditions however where antihistamines are not commonly used. For example, oral antihistamines are not recommended as first line treatment for children with atopic eczema, unless the condition is severe or has urticaria associated with it, in which case a one month long course of a non-sedating antihistamine should be prescribed. ( NICE, 2007) Another use of antihistamines, more specifially cetirizine, is helping to reduce pollen allergy related coughs in children. (Ciprandi et al., 1997) Antihistamines can be administered in a range of different formulations, although the most common is oral, they can also be a topical form or injected intravenously. (Fitzsimons et al., 2014)

However, although they have many clinical benefits, it has also been shown that antihistamines do also result in a myriad of adverse drug reactions, including aggression, headaches and seizures, although these were more commonly reported with first-generation antihistamines. (Allegaert and Choonara, 2016) Some sedating antihistamines also produce a variety of other effects, including an anticholinergic effect, which results in side effects such as blurred vision, dry mouth, dizziness and confusion. (Mintzer and Burns, 2000) To ensure safety, it would be ideal for antihistamines with a sedative effect to be only available on prescription, as many people do not realise that these drugs can have a significant impact on cognition. (Kay, 2000) When choosing between first and second-generation antihistamines, it is widely accepted that second-generation drugs are the safer and more effective choice, as an example, unlike second-generation drugs, first-generation antihistamines are able to cross the blood-brain barrier which results in CNS effects such as drowsiness and seizures. (Anagnostou, Swan and Brough, 2016) In terms of overdose, second-generation antihistamines are considered much safer as overdosing on first-generation drugs can cause cardiac arrest and death, compared to the vomiting and drowsiness associated with an overdose on second-generation drugs. (Anagnostou, Swan and Brough, 2016)

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