Expectorants and Antitussives

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Chapter: Medicinal Chemistry : Expectorants and Antitussives

Antitussives are drugs that reduce coughing. Coughing may be diminished by reducing respiratory secretion, eliminating a source of irritation, or decreasing the sensitivity of irritant receptors within the respiratory tract.


EXPECTORANTS AND ANTITUSSIVES

Antitussives are drugs that reduce coughing. Coughing may be diminished by reducing respiratory secretion, eliminating a source of irritation, or decreasing the sensitivity of irritant receptors within the respiratory tract. Antitussives can act either by raising the threshold of the cough centre or by reducing the number of impulses transmitted to the centre from the peripheral receptors. The antitussives are divided into two main classes:

1.           Centrally active antitussives that affect the cough centre in the medulla.

2.           Peripherally active antitussives that act at the receptor level in the respiratory tract.

Pharyngeal demulcents sooth the throat and reduce the afferent impulse from the inflammed/irritated mucosa. Expectorants are drugs that increase the secretion of bronchus and reduce the viscosity, thereby removing cough and sputum. The drugs usually used as expectorants are salts of sodium, potassium, and ammonium compounds. Guaiphenesin, vasaka, and balsam of tolu are plant products. These also enhance the mucociliary movement. Ammonium salts are nauseating and reflexly increase the respiratory secretion. Antitussives acts centrally and relieve the cough. They act on the cough centre in the medulla oblongata and increase the threshold to cough. These drugs control the cough rather than eliminate it. The drugs that are used as antitussives are codeine, an opium alkaloid, and selective drugs are also there for the action on the cough centre, but it may produce respiratory depression at higher dosage. In non-opioids, noscapine and dextromethorphan are usually used. Noscapine is an opium alkaloid of benzoisoquinoline series. It depresses cough, but has no narcotic, analgesic, or dependence effects. Dextromethorphan is a synthetic compound and it is a d-isomer and used as antitussives.

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