- Introduction.

Antihistamine drugs are the most common medical drugs against allergic reactions of every type and are symptomatic drugs, that limit and block the acute symptoms of these disturbs. In the allergic reaction, the contact with the substance responsible of the disease stimulates the release of histamine (1H-imidazole-4-ethanamine), a substance present in all the body tissues and released by special allergy cells, but particularly in the skin, in the lungs and in the mucosa of the digestive apparatus.

The histamine receptors are structures of the nervous system that allow the transmission of a message and they can be of 3 types:

1) The H1 receptors (mainly present in the skin, in the bronchial tube, in the stomach and in the gastro-intestinal mucosa)

2)The H2 receptors (mainly present in the stomach)

3) The H3 (mainly present in brain).

- Uses and action.

The antihistamine drugs are similar to histamine and they block these histamine receptors at the place of histamine, so preventing this molecule to act in the organism as a chemical messenger and make proceed the allergic reaction. They are used for the treatment of various allergies, like hayfever, featured by terrible and diffused itch.

Moreover, they are useful in the treatment of the a-topic eczema, of the allergic conjunctivitis and rhinitis, of the acute and chronic urticaria, of some allergic dermatitis and in all the diseases where there is a high histamine presence in organs and tissues. The antihistamines seem to be useless in the treatment of the bronchial asthma and in the symptomatic therapy of the common cold.

Histamine has not only responsible in activating an allergic or immune reaction, but have a very important action in our brain to keep us awake and concentrated on what we are doing. So, blocking histamine, we tend to fall asleep and this is just what happened with the old generation of antihistamines that reached the brain, deactivating histamine also where not necessary. This was a dangerous side effect, especially for work and driving. The most recent antihistamines, instead, are not anymore able to reach the brain and are safe, regarding this problem.

- Absorption by the organism.

These drugs are normally taken in tablets, capsules or liquid medicines. They are mainly absorbed in the intestine and in the mouth and their action generally starts after 15-30 minutes, it reaches its maximum effect within 1 hour and ceases after 3-6 hours.
They can be used to reduce the reddening, the oedema, the itch and the bronchial constriction, of course , only if due to histamine release.

- Side effects.

Some cases of anorexia, nausea, vomit, constipation, diarrhoea, gastric diseases, muscular pain, dry mouth, reduced concentration ability, blurred vision (dangerous during driving) have been reported in the medical literature. The old antihistamines, as already reported, can cause frequent sleepiness.

Antihistamines are not recommended in patients with prostate hypertrophy or glaucoma and who are already assuming antibiotics or antifungal drugs, due to dangerous interactions among these substances.
These side effects are present also in the newest antihistamine drugs, (the 2nd generation antihistamines).

References:

http://www.users.globalnet.co. uk/~ aair/antihistamines.htm#WHATRA NTI

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