They have a longer duration of action and longer plasma half-life than the first generationdrugs, which makes it possible to reduce the number of daily intakes.They do not have, in normal dose, antimuscarinic effect, except mequitazine which has a mild antimuscarinic effect.Mequitazine has an antimuscarinic effect parallel to its H1-antihistamine activity, which can result for example in dryness of mouth or accommodation disturbances.
It is used in treatment of chronic urticaria, atopic dermatitis and dermographisms. Doxylamine is a sedating antihistamine also used as an hypnotic.
Histamine-1 receptors are also found in the brain and spinal cord, and stimulation of these receptors makes you more awake and alert.
Sedating antihistamines oppose the effects of histamine on H1 receptors in your brain, which is why they cause sedation and drowsiness.
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.