Histamine, acting on H-antihistamines help against these effects, they work only if taken before contact with the allergen.In severe allergies, such as anaphylaxis or angioedema, these effects may be of life-threatening severity.(There are other types of antihistamines that affect H-2, H-3, and H-4 receptors.) People build up tolerance to them over time and when they stop taking them experience rebound insomnia.
Doxylamine can be used by itself as a sleep aid or combined with other drugs for cold and allergy relief (usually in a night-time formula).
For people who need to get to sleep, however, the drowsiness is the main benefit of antihistamines.
These are usually not prescription drugs and can be purchased at retailers over-the-counter.
Antihistamines make your sleepy by antagonizing central histimine-1 (H-1) receptors in the brain.
The ones you see in over-the-counter sleep aids are usually diphenhydramine and hydroxyzine.