Antihistamines work by blocking the action of histamine which the body produces as part of its natural defences.
Histamine is released as a reaction to foreign substances (allergens), and starts a chain reaction that leads to the release of other chemicals that add to the allergic response. There are two different types of antihistamine; sedating antihistamines, which can enter the brain and cause drowsiness and non-sedating antihistamines which do not.
A short cut review was carried out to establish whether non-sedating antihistamines reduced symptom severity or duration in the common cold.
156 papers were found using the reported searches, of which 4 presented the best evidence to answer the clinical question.
That said, in overdose selectivity is lost and patients can develop increased sedation, anticholinergic effects and hypotension.
QT prolongation occurs secondary to potassium channel blockade.
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The dichotomy between 1st and 2nd generation antihistamines was introduced in 1983 at about the time of the European registration of two drugs: terfenadine and astemizole.This is not an indication of a security issue such as a virus or attack.