Dr. Singh, ACSH friend and former Chief Medical Officer for the Office of the Assistant Secretary for HHS, published a paper on the adulterants found in street drugs. What was in there? A lot of junk.
There are many good reasons to avoid illegal drugs, like addiction and overdosing. The next best is that local street chemists aren't terribly trustworthy. It's common for drugs to be mixed with other chemicals, some of which are far more dangerous than the drug itself. Case in point: Fentanyl, which is often mixed into heroin.