dissolution

Before a swallowed tablet can do anything, its solid drug must turn into dissolved molecules in the watery fluids of the gut, because only dissolved drug can cross into the bloodstream. Dissolution is that handoff: how fast and how completely the solid melts away into solution. Think of a sugar cube in tea — a fine powder vanishes in seconds, a hard lump lingers.

Dissolution is distinct from solubility. Solubility is the maximum amount that can dissolve at equilibrium, a fixed ceiling, while dissolution is the rate at which the solid approaches that ceiling. The rate rises with smaller particles, which expose more surface area, with more soluble solid forms, and with stirring and warmth. For many drugs, especially poorly soluble ones, dissolution is the slowest step in absorption and therefore controls how much and how quickly the drug enters the body.

Because dissolution is rate-limiting so often, it is measured early and routinely, using standardized apparatus that tumbles or stirs the dosage form in a fluid mimicking gastric or intestinal conditions and samples how much has dissolved over time. A dissolution test that correlates with how the drug performs in people becomes a powerful quality-control and development tool. The caveat is that such in vitro–in vivo correlations are not guaranteed and must be established case by case.