solvent

The solvent is the liquid that does the dissolving — the water in your cup of tea, the medium that welcomes the solute and pulls it apart. It is usually the component present in the greatest amount, and it sets the overall character of the solution.

Water is the dominant pharmaceutical solvent because it is cheap, safe and biocompatible; its strongly polar molecules surround and stabilise ions and polar drugs in a process called solvation (specifically hydration when the solvent is water). When water alone cannot dissolve enough drug, formulators add water-miscible cosolvents such as ethanol, propylene glycol or glycerol, or use non-aqueous solvents such as vegetable oils for oily injections.

Choice of solvent is never just about dissolving power. It must be non-toxic at the concentration used, compatible with the drug and the container, acceptable by the intended route, and not so volatile or reactive that it harms stability. Pharmacopoeias define purified water, water for injection and other solvent grades precisely for these reasons.