That would help stop people from taking things that are adulterated, but I'd be concerned about the toxicity of the pure drugs as well.

A great example of what can happen is Fen-Phen, the diet drug that was taken off the market a while back. The drug fenfluramine also activates the 5-HT2B receptor and causes heart valve damage.

All of these designer drugs out there could certainly have off-target effects that would never be noticed unless a clinical trial was run (even then maybe not!) or unless enough users have enough problems that it's noticed (MPTP is great example).