Sedating antihistamines effect is related to
Antihistamines can cause drowsiness, which might help you fall asleep for a few nights — but routine use of antihistamines for insomnia isn't recommended.
Although many findings provide reassurance about the relative safety of many antihistamine drugs and that any malformation reported is most probably caused by chance, studies are still required to assure fetal safety.Tolerance to the sedative effects of antihistamines can develop quickly.As a result, the longer you take them, the less likely they are to make you sleepy.In addition to this antihistamine effect, they can have parallel pharmacological properties, for example antimuscarinic and adrenolytic effects which must be considered.It is usual to classify H1-antihistamines into two classes: old ones, called first-generation agents, which are sedating and recent ones, called second-generation agents which have slight or no sedating effects because they do not cross the blood-brain barrier.The drugs of the first antihistamine generation are: promethazine, alimemazine, dexchlorpheniramine, brompheniramine, buclizine, carbinoxamine and doxylamine, are sedating and elicit a drowsiness which can be awkward.
The first-genaration antihistamines have alpha adrenolytic activity which can decrease the vasoconstrictive effect of adrenaline and noradrenaline and an antimuscarinic effect with the corresponding adverse effects.
They do not always completely control the allergic reaction because they do not counteract other chemicals that may be responsible for the symptoms.
There are at least two kinds of histamine receptors, hence are mainly H1 blockers.
It is hardly surprising, therefore, that these first-generation antihistamines had poor receptor selectivity and significant unwanted side effects.
During this time, knowledge of the nature and diversity of receptors was rudimentary to say the least and it was only several decades later that the existence of more than one species of histamine receptor was discovered.
In fact, most over-the-counter sleep aids contain antihistamines.