cocrystal

A cocrystal is a single crystal that politely shares space between two different molecules: the drug and a second, harmless partner molecule called a coformer. Think of two dance partners locked into one repeating choreography across the whole crystal. Both keep their own chemical identity, but together they form a new solid with properties neither has alone.

Unlike a salt, where a proton is transferred and the components carry opposite charges, a cocrystal holds its two neutral components together mainly through non-ionic interactions such as hydrogen bonds. This makes cocrystals especially valuable for drugs that are not ionizable and therefore cannot form salts. By choosing a coformer wisely, chemists can tune solubility, dissolution rate, stability, and even how easily the powder flows and compresses into tablets.

The coformer is usually selected from a list of generally safe substances, often simple organic acids, sugars, or amino acids, so the cocrystal does not add new toxicity concerns. The main challenges are that a suitable coformer must actually co-crystallize with the drug, which is not guaranteed, and that the cocrystal must be reliably reproducible at manufacturing scale and stable against splitting back into its separate components.