I tried doing some research on this and could not come up with anything stating a rider gets drug tested. I would asume, however, if the rider was acting dangerous while riding, then maybe a test would be in order.

On-the-track however, it states that it is a requirment that all exercise riders and jockeys be tested once a year, but I have never seen that inforced. All suspious behavor, they will drug test at the time and if tested positive, that rider will be rulled off of every track for either a year or lifetime depending on what the Stewart decides.