Alimemazine and promethazine are considered to be the most sedating first-generation antihistamines, whereas cyclizine and chlorphenamine are considered the least sedating of this group, according to Patient.co.uk. Other members of this class of drugs include clemastine, hydroxyzine and cyproheptadine.
First-generation antihistamines are associated with sedating side effects, whereas second-generation antihistamines, such as loratadine and fexofenadine, cause sedation to a lesser degree, explains Patient.co.uk. This is because first-generation antihistamines are more lipid soluble, meaning that they cross the blood-brain barrier to a greater extent.
While the sedation can be frustrating for some patients, it can have beneficial effects in treating some conditions, such as urticaria or atopic dermatitis, notes Patient.co.uk. Sleep is often disturbed due to irritation, meaning sedation can offer some symptomatic relief by way of inducing sleep in affected patients. However, first-generation antihistamines are also associated with side effects such as confusion, urinary retention and dry mouth.
First-generation antihistamines are used to treat conditions such as allergic rhinitis, urticaria, asthma, nausea, vertigo and cough, according to Patient.co.uk. Patients are advised not to consume alcohol while taking first-generation antihistamines, as it increases sedative effects. The effects of drugs in this class derive from their activity at H1 receptors in the central nervous system, while many of the side effects of first-generation antihistamines result from their activity of antagonising muscarinic acetylcholine receptors.