crystal form

Picture a parking lot where every car is parked in a neat, repeating grid. That orderly, repeating pattern is what makes a solid crystalline, and the specific pattern a drug's molecules choose is its crystal form. The same drug can have several possible crystal forms, and each one is a real, physically different solid with its own behavior.

Crystal form is an umbrella term covering all the ordered solid states a compound can adopt: different polymorphs of the pure substance, salts with their counterions, cocrystals with a coformer, and solvates or hydrates that trap solvent or water in the lattice. Each form has a defining internal arrangement, detectable by X-ray diffraction, that fixes its melting point, density, mechanical properties, and how readily it releases molecules into solution.

Because these properties feed directly into solubility, stability, and manufacturability, choosing and locking in a single, reproducible crystal form is one of the most consequential decisions in turning a molecule into a medicine. A form that is easy to make but unstable, or stable but poorly soluble, can sink a program. The chosen form is also a key part of a drug's patent and regulatory filing, since competitors may try to use a different crystal form.