The solvents usually used in the oral liquid preparations are purified water, alcohol, glycerin and propylene glycol.

Purified water: Naturally occurring water exerts its solvent effect on most of the drug substances. In oral preparations, demineralized water or purified water is used.

Alcohol: This is the most useful primary solvent for many organic compounds. It acts as a cosolvent and increases the solubility of drugs. Alcohol is preferred because of its miscibility with water and its ability to dissolve many water-insoluble ingredients. It is also used in liquid products either as an antimicrobial preservative alone or as a co-preservative with parabens, benzoates, sorbates and other agents.

Glycerin: It is a clear syrupy liquid with a sweet taste. It is miscible with water and alcohol. It has preservative qualities. However, because of its viscosity, solutes are slowly soluble in it unless it is rendered less viscous by heating.

Propylene glycol: It is a viscous liquid and is miscible with water and alcohol. It is a useful solvent with a wide range of applications and is frequently substituted for glycerin in pharmaceutical formulation.