Antitussives that act peripherally
When the cough arises above the larynx, syrups and lozenges that glutinously and soothingly coat the pharynx (demulcents) may be used, e.g. simple linctus (mainly sugar-based syrup). Small children are prone to swallow lozenges and so a confection on a stick may be preferred.
Linctuses are demulcent preparations that can be used alone and as vehicles of other specific antitussive agents. That their exact constitution is not critical was known and taught to medical students in 1896.
Many of you know that this (simple) linctus used to be very much thicker than it is now, and very likely the thicker linctus was more efficacious. The reason why it was made thinner was this. It was discovered that a large number of children came to the surgery complaining of cough, and they were given the linctus, but instead of their using it as a medicine, they took it to an old woman out in Smithfield, who gave them each a penny, took their linctus, and made jam tarts with it.2
When cough arises below the larynx water aerosol inhalations and a warm environment often give relief — the archetypal 'steam' inhalation. Com
2 Brunton L 1897 Lectures on the action of medicines.
pound benzoin tincture3 is often used to give the inhalation a therapeutic smell (aromatic inhalation). This manoeuvre may have more than a placebo effect by promoting secretion of a dilute mucus that gives a protective coating to the inflamed mucous membrane. Menthol and eucalyptus are alternatives.
Local anaesthetics can also be used topically in the airways to block the mucosal cough receptors (modified stretch receptors and C fibre endings) directly. Nebulised lignocaine, for example, reduces coughing during fibreoptic bronchoscopy and is also effective in the intractable cough that may accompany bronchial carcinoma.
The most consistent means of suppressing cough irrespective of its cause is blockade of the medullary cough centre itself. Opioids, such as methadone and codeine, are very effective although part of this antitussive effect could reflect their sedatory effect on higher nervous centres; nevertheless antitussive potency of an opiate is generally poorly correlated with its potency at causing respiratory depression. There are also nonopioid targets too, since dextromethorphan (the d-isomer of the codeine analogue levorphanol) and pholcodine have an antitussive effect that is not blocked by naloxone. These opiates also have no significant analgesic or respiratory-depressant effects at the doses required for their antitussive action confirming that opiate receptors are not involved.
Opioids are usually formulated as linctuses for antitussive use. Deciding on which agent to use depends largely on whether sedation and analgesia may be useful actions of the linctus. Hence methadone or diamorphine linctus may be preferred in patients with advanced bronchial carcinoma. In contrast, pholcodine, being nonsedating and nonaddictive, is widely incorporated into over-the-counter linctuses.
Sedation generally reduces the sensitivity of the cough reflex. Hence older sedating antihistamines, e.g. diphenhydramine, can suppress cough by non-Hj-receptor actions; often the doses needed cause substantial drowsiness so that combination with other drugs, such as pholcodine and dextromethorphan, is common in over-the-counter cough remedies.
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