Drugs containing amine groups react with CO₂ to form crystalline ammonium carbamates or carbamic acids. In this approach, both the cation and anion of the salt, or the neutral CO₂ adduct, are derived from the parent drug, generating new crystalline versions in a ‘masked’ or prodrug form. It is proposed that this approach may serve as a valuable new tool in engineering the physical properties of drugs for formulation purposes.