Dextromethorphan hydrobromide (DXM or DM) is the active ingredient in many cough medicines, famously sold as Robitussin, working as a cough suppressant (antitussive) to calm the cough reflex, often combined with other drugs like guaifenesin (an expectorant) in formulations like Robitussin DM for multi-symptom relief of cough, congestion, and chest buildup, available in syrups (liquid), gels (cough gels), and lozenges (medi-soothers). While effective and generally safe when used as directed, misuse is possible, and it's important to follow label instructions and consult a doctor for conditions like asthma or liver disease.

The determination of the specific rotation of dextromethorphan HBr is done for several important reasons, mainly related to identity, purity, and quality control. Its levo isomer, however, is a narcotic.

Any contamination of the medication by the narcotic would have dangerous consequences and so its isomeric purity is extremely important. USP (United States Pharmacopeia) recommends that this purity be checked by measuring optical rotation, as it is the only convenient means.

Confirmation of optical activity and identity; Dextromethorphan HBr is a chiral compound. It rotates planepolarized light in a specific direction and by a specific amount. Measuring its specific rotation helps confirm that the substance is dextro-rotatory dextromethorphan, not its levo isomer or a different compound.

In this experiment, dextromethorphan hydrobromide was measured using a P-2000 polarimeter with a halogen lamp and interference filter at 325 nm. Specific Rotation specification is within ±1% of USP Std., 250 nominal. Specific rotation, determined photoelectrically at 325 nm, does not differ from the similarly prepared solution of USP Dextromethorphan Hydrobromide RS by more than 1.0%, barely met the requirements.