dissolution rate

Dissolution rate is simply the speed of dissolving: how much drug enters solution each minute. Two sugar cubes may eventually fully dissolve, but crushed sugar dissolves much faster — same amount in the end, very different rate.

Quantitatively, dissolution rate is the mass of solid passing into solution per unit time, often expressed as milligrams dissolved per minute. The Noyes-Whitney equation predicts it: the rate rises with the surface area exposed, with the difference between solubility and the current dissolved concentration (the driving force), and with stirring, and falls as the diffusion layer thickens.

Because dissolution can be the slowest step before absorption, controlling dissolution rate is a central formulation lever. Speeding it up — by micronising the drug, using a salt or an amorphous form, or adding a disintegrant — helps poorly soluble drugs work faster. Slowing it down deliberately — with coatings or polymer matrices — is how sustained- and controlled-release products spread a dose over many hours.