| The afferent arc of the cough reflex receives input from sensory nerves in the bronchial and tracheal airways. Airway irritation and inflammation stimulate the afferent nerves, which in turn activate the cough center located in the medulla oblongata. Most of the antitussive drugs are opiates or opioids that directly suppress the cough center in the medulla oblongata (Table:
Antitussive Drugs). The antitussive effect does not appear to be related to the binding of
traditional opiate receptors. For example, dextromethorphan is an opiate derivative with good antitussive activity, but it does not have activity at opiate receptors and is not analgesic or addictive. |
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Morphine is an effective antitussive at doses lower than the doses that produce analgesia and sedation. It is not commonly used for antitussive activity due to side effects and the potential for abuse and addiction. Morphine has poor oral bioavailability due to a significant first-pass effect by the liver. |
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Codeine is methylmorphine; methylation of morphine significantly improves the oral bioavailability by reducing the first-pass effect. Codeine phosphate and codeine sulfate are found in many preparations, including tablets, liquids, and syrups. Codeine has analgesic effects that are about one-tenth that of morphine, but its antitussive potency is about equal to that of morphine. The side effects of codeine are significantly less than those seen with morphine at
antitussive doses. Toxicity (especially in cats) is exhibited as excitement, muscular spasms, convulsions, respiratory depression, sedation, and constipation. Codeine should not be used after GI tract surgery. The potential for addiction and abuse of codeine is considerably lower than that for morphine. |
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Hydrocodone is chemically and pharmacologically similar to codeine but more potent. It is combined with an anticholinergic drug (homatropine) to discourage abuse by people. It can be prescribed for small animals but should be used with caution in cats. |
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Dextromethorphan is technically not considered an opiate because it does not bind to traditional opiate receptors and is not addictive or analgesic. It is the d-isomer of levorphanol. The l-isomer of levorphanol has addictive and analgesic properties. Dextromethorphan is the safest antitussive to use in cats and is reported to be more efficacious in cats than codeine. |
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Butorphanol, an opioid agonist-antagonist, is used as an analgesic and antitussive in dogs. It is more potent than morphine as an analgesic and more potent than codeine or dextromethorphan as an antitussive. It may produce considerable sedation. Because butorphanol has poor bioavailability, the oral dose in dogs is 10 times the SC dose. Its use in cats is controversial. |
