Source: Medicines and Medical Devices Safety Authority (NZ) Revision Year: 2023 Publisher: Pfizer New Zealand Limited, P O Box 3998, AUCKLAND 1140, New Zealand, Toll Free: 0800 447 400, Web: www.robitussin.co.nz
Guaifenesin is an expectorant, which clears chest congestion by loosening and reducing the viscosity of phlegm, increasing the volume of phlegm and making coughs more productive.
Dextromethorphan is a non-opioid cough suppressant. It is the methylated dextrorotatory analogue of levorphanol, a codeine analogue. Dextromethorphan acts centrally on the cough centre in the medulla and nucleus tractus solaris to increase the cough threshold. It does not have classical analgesic, sedative or respiratory depressant effects at usual antitussive doses.
Guaifenesin is well absorbed from the gastrointestinal tract. It is metabolised primarily to Beta-2-methoxyphenoxy-lactic acid and then excreted in the urine. In a study where subjects were given 400 mg of guaifenesin orally, no unchanged drug could be detected in the urine. The plasma half-life of guaifenesin is one hour.
Dextromethorphan is well absorbed from the gastrointestinal tract after oral administration. It is metabolised in the liver, exhibiting polymorphic metabolism involving the cytochrome P450 isoenzyme (CYP 2D6). It is excreted in the urine as unchanged dextromethorphan and demethylated metabolites, including dextrorphan, which has some cough suppressant activity. The plasma elimination half-life of dextromethorphan is 1.2 to 3.9 hours. However, the rate of metabolism varies between individuals according to phenotype (extensive v poor metabolisers), with half-life being as long as 45 hours in patients who are poor metabolisers.
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