excipient
A tablet is mostly not the drug. The active ingredient might be a tiny pinch of powder, far too little to handle, shape, or swallow on its own. Everything else in the pill — the bulk that gives it size, the glue that holds it together, the coating that makes it slide down — is the excipient. Excipients are the supporting cast that turns a raw drug substance into a usable product.
Excipients are, by definition, intended to be pharmacologically inactive, but they are far from inert in a practical sense. They serve specific jobs: fillers add bulk, binders hold particles together, disintegrants make the tablet break apart in the gut, lubricants keep powder from sticking to machines, and coatings control taste, appearance, or where the drug is released. The right choices can speed or slow dissolution and protect the drug from moisture and light.
Because excipients are not biologically silent, they must be chosen carefully. Some patients react to specific excipients such as lactose or certain dyes, and an excipient can chemically interact with the drug to speed its degradation, a problem called incompatibility. Formulators run compatibility studies and select excipients with a long record of safe use at the intended amounts.
Although called inactive, some excipients are functional enablers: surfactants and solubilizers can dramatically boost the absorption of a poorly soluble drug, blurring the old line between active and inactive.