Why is alcohol used in cough syrups?

Most cough syrups contain menthol and natural extracts, for carminative and antitussive actions. These substances are water-insoluble and so alcohol is used as a solvent.

Alcohol is miscible in all proportions with many of the syrups and so is the solvent of choice for many preparations. This property is rendered by its R-OH (hydroxyl) group which increases miscibility via inter molecular hydrogen bonding with solute particles. This enables the syrup to be absorbed easily by the body. It is used as a preservative in pharmaceutical preparations to prevent microbial growth. In addition syrups are mainly made of gels which in turn contain a liquid (as dispersed phase) and a solid (as dispersion medium). In them, alcohols play the role of stabilizers by preventing coagulation of solute particles during storage.

Sucrose is used as a vehicle in most of the medicated syrups. If the syrup is completely saturated with sucrose, under cool storage conditions some sucrose might crystallize from the esolution and, by acting as nuclei, initiate a type of chain reaction that would result in the separation of an amount of sucrose disproportionate to its solubility at the storage temperature. The syrup would then be very much unsaturated and probably support microbial growth. Many of them are not intended to be as nearly saturated as simple syrup (85g of sucrose is dissolved in 100 ml of purified water) and therefore need preservatives to ensure their stability   during storage. Sucrose may be substituted in whole e or in part by other agents in the preparation of medicated syrups. Polyols like sorbitol or a mixture of polyols like sorbitol and glycerin are commonly used. Here, alcohol serves as a preservative and stabilizer.

Alcohols are good sedatives and hypnotics as they depress the activity of the central not nervous system and lead to sleep. As a result they reduce the irritation in the throat and also reduce coughing.

Hypnotic property of alcohols increase with increase in molecular weight. Heavy doses of some of these alcohols can lead to anesthesia and coma.