Drugs Used Predominantly as Antitussives
Certain opioids are used mainly for their antitussive ef-fects. Such drugs generally are those with substituents on the phenolic hydroxyl group of the morphine struc-ture. The larger the substituent, the greater the antitus-sive versus analgesic selectively of the drugs.
Dextromethorphan hydrobromide is the D-isomer of levorphanol. It lacks CNS activity but acts at the cough center in the medulla to produce an antitussive effect. It is half as potent as codeine as an antitussive. Anecdotal reports of abuse exist, but studies of abuse potential are lacking. It has few side effects but does potentiate the activity of monoamine oxidase inhibitors, leading to hy-potension and infrequently coma. Dextromethorphan is often combined in lozenges with the local anesthetic benzocaine, which blocks pain from throat irritation due to coughing.
Levopropoxyphene is the L-isomer of the analgesic ago-nist dextropropoxyphene. Levopropoxyphene is only mildly antitussive and is rarely used. It has no CNS ef-fects. Side effects include dizziness and nausea. It is available as the napsylate derivative (Novrad) and is taken orally in the form of a liquid or less frequently as a capsule.
Noscapine is a naturally occurring product of the opium poppy. It is a benzylisoquinoline with no analgesic or other CNS effects. Its antitussive effects are weak, but it is used in combination with other agents in mixtures for cough relief.
Benzonatate (TessaIon) is related to the local anes-thetic tetracaine. It anesthetizes the stretch receptors in the lungs, thereby reducing coughing. Adverse reactions include hypersensitivity, sedation, dizziness, and nausea.