Biologic drugs are larger and more complex than other pharmaceuticals, because they're built by living organisms--usually cells in a petri dish. Pharmaceutical companies typically patent those cellular factories, so generic companies have to use slightly different cells to make their drugs. The resulting medicines are similar in composition, but not identical--hence the term 'biosimilar'. Until now, biologic drugs have faced no generic threats because there wasn't a clear way to regulate the copycats. The FDA said in a news release that biosimilars must have "no clinically meaningful differences in terms of safety and effectiveness from the reference product." But because the drugs can't be completely identical, the agency allows for minor differences in inactive compounds.