This is also interesting. From a pdf on a US based racing medications website.
Of the four substances, namely testosterone, boldenone, nandrolone, and stanzolol, the first three are naturally occurring, also known as endogenous, in the horse.
Testosterone is present in intact males, gelded males, and fillies while nandrolone and boldenone are naturally occurring in both intact and gelded males. As with all drugs, withdrawal times are specific to the formulation.
As a group, anabolic steroids stay around for a long time, longer than any other group of drugs used in racing. Trainers should be looking at a minimum of 30 days with the water-based anabolic steroids, such as stanozolol (Winstrol) and aqueous testosterone.
The oil-based anabolic steroids, such as Equipoise
boldenone), Durabolin (nandrolone) and testosterone, will need a minimum of 45 days.