Pharmacology: Dextromethorphan HBr is a cough suppressant which reduces cough frequency. Glyceryl guaiacolate is used as an expectorant and has a sputum-thinning effect. Addition of chlorpheniramine maleate is useful in case of cough due to allergy. The sedative side effect of chlorpheniramine maleate has an advantage in the treatment of cough.
Pharmacokinetics: Following oral administration, dextromethorphan is well absorbed. The peak plasma concentration is reached 2 hrs after administration, whereas the maximum clinical effect occurs 5-6 hrs after ingestion of dextromethorphan tablets.
Dextromethorphan is extensively metabolized in the liver and the main metabolite is dextrorphan. About 50% of a dose is excreted in the urine over 24 hrs. Less than 1% of the dose is excreted in the feces. About 8% of the dose is excreted as unchanged drug in the urine in 6 hrs. It is not known if dextromethorphan or dextrorphan is excreted in breast milk or crosses the placenta.
Chlorpheniramine maleate is well absorbed after oral administration and distributed into the tissues. Chlorpheniramine maleate appears to undergo moderate first-pass metabolism and there may be an enterohepatic circulation.
Considerable intersubject variation has been found in the metabolism and excretion of chlorpheniramine maleate and the excretion was dependent on urinary pH and flow rate. About 35% of a dose was excreted as unchanged, 22% as the desmethyl metabolite and 3-10% as didesmethyl metabolite. None of the metabolites are toxic or responsible for clinical antihistaminic effect. In children, there was evidence to suggest more rapid and complete oral absorption and faster clearance.
Glyceryl guaiacolate is readily absorbed from the gastrointestinal tract, with plasma half-life of approximately 1 hr. Glyceryl guaiacolate is rapidly metabolized by oxidation to β-(2-methoxy-phenoxy)lactic acid. About 40% of a dose is excreted as metabolite in the urine in 3 hrs.