Antihistamines can cause drowsiness, which might help you fall asleep for a few nights — but routine use of antihistamines for insomnia isn't recommended.
H1-antihistamines have been used for more than fifty years in treating various allergic manifestations. H1-antihistamines which penetrate into brain elicit, by inhibiting stimulant effect of histamine, drowsiness.
Side effects might include daytime drowsiness, dry mouth and dizziness.
Also, diphenhydramine and doxylamine — sedating antihistamines often found in over-the-counter sleep aids — aren't recommended for people who have certain conditions, such as closed-angle glaucoma, asthma, chronic obstructive pulmonary disease or severe liver disease.
In addition, each compound can have or not have parallel properties, antimuscarinic effects for example.
This distinction between generations must be taken with caution because a product considered as non sedative or not antimuscarinic can, in certain circumstances, large doses or particular susceptibility of the patient ,have these effects.
There are two kinds of histamine receptors, H1 and H2.