polymorph

The same identical molecules can stack into a crystal in more than one way, like the same bricks arranged into two different walls. Each arrangement is a polymorph. Although the molecule is chemically unchanged, the packing differs, and that difference quietly governs how fast the solid dissolves, how stable it is, and how easily it can be pressed into a tablet.

Polymorphs differ in the geometry and energy of their crystal lattices, so they show different melting points, densities, and solubilities even though they are the same substance. One polymorph is usually the most thermodynamically stable at a given temperature and will, over time, tend to convert to it; the others are metastable and can persist for a long time or transform unexpectedly. Detecting and characterizing every accessible form is a core task of solid-state chemistry, using tools such as X-ray powder diffraction.

Polymorphism matters because an unintended switch to a more stable, less soluble form can sharply reduce a drug's dissolution and absorption, sometimes after a product is already on the market. The classic cautionary tale is ritonavir, where a previously unseen, far less soluble polymorph appeared during manufacturing and forced the capsule formulation to be withdrawn and reworked. Regulators therefore expect a thorough polymorph screen during development.