Just to be clear on what is considered doping, the WADA considers any substance (or method) to be on the prohibited list if it meets two of the following 3 criteria[1]:

- It has the potential to enhance or enhances sport performance - It represents an actual or potential health risk to the athlete - It violates the spirit of sport

So, in theory, the distinction of acceptable and non-acceptable is not drawn (only) in performance-enhancing drugs.

Another thing to consider is that (pre-BALCO) you are not always charged on taking performance-enhancing drugs but in either having some synthetic drugs on your organism detected or having some measurements over what is considered "normal" in humans. For example, T/E higher than 4:1 ratio. This means that the ratio Testosterone/Epitestosterone can be 4:1. If an athlete has a 1:1 level (which is normal), doping himself with testosterone won't be considered positive as long as it's kept below 4:1. All assuming that exogenous testosterone cannot be detected.

As far as I know, BALCO was the first time than some athletes were sanctioned without testing positive.

[1] http://www.usada.org/substances/prohibited-list/ [2] http://www.webmd.com/fitness-exercise/steroid-doping-questio...