But a high-profile case has brought the safety of this practice under scrutiny.

When 21 polo horses died suddenly in Florida this April, an investigation revealed a fatal error in the amount of selenium contained in a compounded vitamin-mineral supplement. Many similar mishaps have occurred, including a situation in Louisiana in 2008, when several horses died after receiving a compounded clenbuterol product.

So what is a compounded drug and how can a trainer be sure to avoid a potential error in compounding?