polymorphism

The same Lego bricks can be assembled into a flat tile or a tall tower — same pieces, different arrangement. Polymorphism is when identical drug molecules pack into the crystal lattice in more than one distinct way. Each arrangement, called a polymorph, is chemically the same substance but a physically different solid.

Because the molecules sit in different lattice geometries, polymorphs can differ in melting point, density, solubility, dissolution rate, mechanical behavior and stability — sometimes dramatically. Usually only one polymorph is thermodynamically most stable at a given temperature; the others are metastable and may convert to the stable form over time, especially under heat, humidity or mechanical stress during milling or compression.

This matters intensely for medicines. A more soluble metastable polymorph might give better absorption but could revert in the bottle, changing the dose the patient actually receives. Regulators expect manufacturers to identify all relevant polymorphs, control which one is made, and confirm it stays put. The classic cautionary tale is ritonavir, whose unexpected new, less soluble polymorph appeared years after launch and forced reformulation.