When the patents for medicines expire, other companies are free to make copies of them. For small molecule medicines, these copies are known as generics. Because it is easy to determine their exact structure and how they are made, IDENTICAL copies can be made by following a series of chemical reactions.

For biologics, it’s not as easy. To begin with, because of their size and complexity, it is not possible to exactly determine the structure. Second, the living cells and manufacturing processes used to make the biologic are unique and proprietary to each manufacturer; a different company will not make the biologic in exactly the same way. Copies of biologics and SIMILAR but not identical to the original biologic, hence the name: biosimilars. Clinical testing is important to identify if any of the small differences between the two have an effect in the body.