dissolution

Dissolution is the process of a solid drug dissolving into the surrounding fluid — turning a packed tablet or powder into individual molecules floating in the watery contents of the gut. It matters because a drug can only be absorbed once it is dissolved; a particle that never dissolves simply passes through and is excreted unchanged.

For most oral solid medicines, the sequence is: the tablet disintegrates into granules, the granules break into fine particles, and those particles dissolve. The drug must be in solution before it can diffuse across the gut wall. Dissolution speed depends on the drug's water solubility, the particle size (smaller particles dissolve faster because of greater surface area), the formulation, and the surrounding fluid and pH.

For poorly water-soluble drugs, dissolution is often the slowest step in the whole absorption process — the rate-limiting step. When that is the case, anything that speeds dissolution (milling the drug to tiny particles, using a more soluble salt form, or a liquid rather than a tablet) speeds the onset and can increase the amount absorbed. Conversely, dissolution can be deliberately slowed in modified-release products to spread absorption over time.

Because dissolution governs absorption for many drugs, it is a key quality test: two products with the same drug content but different dissolution behaviour may not be bioequivalent, behaving differently in patients despite identical labels.