Interchangeability of drug products has very different features with small molecules and with biologicals. With small-molecule drugs, a statement of bioequivalence generally indicates therapeutic equivalence and interchangeability. In contrast, with the much more sensitive and complicated biological drugs, a declaration of biosimilarity emphatically does not imply that a patient could be switched from one product to another. Both formulations may be prescribed and administered to subjects who have not received yet the drug in any of its forms. However, regulatory agencies have been very cautious about enabling and permitting interchangeability. Notably, the Biologics Price Competition and Innovation Act of the USA sets very formidable and severe conditions for enabling the interchangeability of biological drug products. The background and conditions for the interchangeability of both small-molecule and biologic drug products are presented in detail. Copyright © 2012 John Wiley & Sons, Ltd.