excipient

An excipient is everything in a medicine that is not the active drug: the bulk that makes a tiny dose into a swallowable tablet, the binder that holds it together, the sugar that hides a bitter taste, the preservative that stops it spoiling. They are often called inactive ingredients, though that name undersells their importance.

Excipients perform specific jobs. Diluents add bulk, binders give cohesion, disintegrants help a tablet break apart, lubricants stop powders sticking to machinery, preservatives prevent microbial growth, and surfactants help insoluble drugs dissolve. A single tablet may contain half a dozen of them, each chosen deliberately.

Though called inactive, excipients are not inert in every sense. Lactose can trouble the lactose-intolerant, some dyes provoke reactions, and an excipient can chemically interact with the drug. Pharmaceutics therefore tests drug-excipient compatibility carefully, and excipients themselves must meet pharmacopoeial quality standards.