This month, the FDA approved a new drug called Inflectra, which could be used to treat a handful of inflammatory conditions such as Crohn's disease, psoriasis, and arthritis. That in itself might not be so surprising—the FDA approves new drugs all the time. But this is only the second biosimilar drug to go on the market in history.

What does that mean?

The first thing to understand is the difference between small-molecule drugs and biologics. Most drugs fall into these two categories, though the distinction is probably most important to the companies making the drugs, to doctors considering their side effects, and to pharmacists, who have to store some of them under special conditions.