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An antihistamine is a drug of different chemical nature that has the effect of countering the action of ‘histamine, one of the main leaders of Allergy.

H 1 Antihistamines

The first, acting on H 1 receptors, causing:

* vasodilatation of the smooth muscle of the bronchi;
* decreased capillary permeability and the formation of edema and ponfi;
* central nervous system depression, in particular ‘ethanolamine.

They are used in the treatment of various reactions of immediate type hypersensitivity (allergies) such as:

* rinite ; rhinitis;
* orticaria ; urticaria;
* congiuntivite . conjunctivitis.

The side effect that occurs in most cases is the sedation, but may also occur dizziness, fatigue and blurred vision.

H 2 Antihistamines

The latter are highly selective and have a zero or minimal effect on H 1 receptor or other types of receptors.

The side effects are minor, given the limited role of H 2 receptor in organs other than the stomach, but also for the low penetration of these drugs across the blood-brain barrier.

The most important molecules as active receptor antagonists H 2 are:

* l ranitidine;
* cimetidine;
*the famotidine.