diluent

A diluent, or filler, is bulk added to a tablet or capsule simply to make it big enough to handle. When a drug dose is only a milligram or two, you cannot make a swallowable tablet from such a speck of powder, so an inert filler is added to bring the weight up to a practical size, much like padding a small gift into a normal-sized box.

Beyond adding volume, a good diluent does extra work: it helps the blend flow, compress and stay uniform. Common choices include lactose, microcrystalline cellulose, dibasic calcium phosphate, mannitol and starch. Some, like microcrystalline cellulose, also bind well and are favoured in direct compression; others, like mannitol, taste pleasantly sweet and cool for chewable tablets.

Diluents are meant to be inert, but they are not truly inactive: lactose can react with amine drugs (a browning reaction), and a few patients are lactose-intolerant, so the filler must be matched to both the drug and the patient.