cocrystal

If a salt is a marriage between an acid and a base that exchange a charge, a cocrystal is more like roommates: the drug shares a single crystal lattice with a second neutral molecule, called a coformer, held together by non-ionic interactions such as hydrogen bonds — no proton fully transfers between them.

Because the coformer is a separate, generally safe molecule (often a food-grade acid, amino acid or other GRAS substance), a cocrystal lets formulators tune properties even for neutral drugs that cannot form a salt. Choosing the right coformer can raise solubility and dissolution rate, improve stability or hygroscopicity, and change mechanical behavior, all while keeping the drug chemically unchanged.

Cocrystals are a defined solid form with their own X-ray powder diffraction pattern, melting point and stability, distinct from the pure drug, its polymorphs and its salts. Regulators treat a well-characterized cocrystal as a distinct drug-product intermediate. The line between a salt and a cocrystal is the degree of proton transfer, which can be ambiguous in borderline acid-base pairs and is probed by techniques like X-ray crystallography and spectroscopy.