Antihistamines are major class of medications prescribed for allergies. Before going into how antihistamines work, let us understand the action of histamines in our body. Histamines play an important role in regulating the immune response. Physiologic effects of histamines on major organs include
1. Contraction of bronchial smooth muscles resulting in wheezing and bronchospasm.

2. Dilatation of small venules and constricting of larger vessels, causing erythema, edema and urticaria
3. Increase in secretion of gastric and mucosal cells resulting in diarrhea.

Histamines act on organs through two types of receptors: H1 and H2 receptors. H1 receptors are predominantly found on bronchiolar and vascular smooth muscle cells. H2 receptors are found on gastric parietal cells. Antihistamines are categorized by these receptors.

Antihistamines are now classified as H1 receptor antagonist and H2 receptor antagonist. H1 antagonists have no effects on H2 receptors. The H1-receptor antagonists or H1-blockers are used in the management of mild allergic disorders. H1-blockers binds selectively to H1 receptors and prevent the action of histamine to these sites. Benadryl is a good example of an antihistamine medication which displays an affinity for H1 receptors. The H2-receptor antagonists are used to treat gastric and duodenal ulcers. Cimentidine targets H2 receptors to inhibit gastric secretions in peptic ulcer disease.

Oral antihistamines are readily absorbed. They are most effective when they are given at the first occurrence of symptoms because they prevent the development of new symptoms by blocking the actions of histamine at the H1 receptors. They are found to be very effective in patients with hay fever, vasomotor rhinitis, hives and mild asthma. The major side effect of all antihistamines is sedation.

Newer antihistamines are called second-generation or non-sedative H1 receptor antagonists. They do not cross blood brain barrier and do not bind to cholenergic, serotonin or alpha adrenergic receptors. They bind to peripheral rather than central nervous system H1 receptors, causing less sedation. They are more costlier than the traditional antihistamines and are no more potent than their predecessors.

Apart from sedation, there are additional side effects for both traditional and second generation antihistamines which includes nervousness, tremors, headache, dizziness, dryness of mouth, palpitations, cardiac dysrhythmias, anorexia, nausea and vomiting. Hence, all antihistamines should be taken under medical supervision.

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