One pill makes you larger, and one pill makes you small A Tensor guide to recreational/illegal drugs and you!
Okay, we've got two potential black market drugs available, and it's just a matter of how long they've been around and how well known they are.
For starters, we've got a drug which was under development by the pharmaceutical industry with an aim of helping 'late bloomers' to manifest powers. Years of R&D, though, yielded a drug which only serves its intended purpose in limited cases and generally with such catastrophic results that research on this particular chemical formula was discontinued entirely. The formula, of course, did not die a quiet, natural death and has been fairly accurately duplicated for black market sale. While it does, in fact, sometimes catalyze mutation, it does so rapidly, painfully, and frequently dangerously. Its 'side effects' are, in fact, more the reason that it is becoming popular. Said side effects being: it will catalyze temporary, random powers in non-powered individuals. The effects last for anywhere from two to twenty-four hours, with the actual duration being difficult to impossible to predict. When taken by already powered individuals it has been known to augment their existing powers, depower them entirely, provide them with an entirely different powerset temporarily...or simply make them intensely physically ill. There is no way to predict the outcome.
It only appeared on the 'scene' approximately three months ago, and has been slowly becoming more popular. Prior to the Smithsonian tragedy it had been linked to a handful of incidents of catastrophic manifestation and has resulted in one publicly known fatality. It has so far caused no known deaths among the already powered simply through administration. While it is not incredibly easy or cheap to get your hands on it is possible.
Our second contestant is a drug with a significantly more nefarious purpose. It was developed very specifically for the black market and is generally used by a more criminal element than the first. Its explicit purpose is to negate powers entirely, and it has been used in a growing number of kidnappings, attempted kidnappings, murders and sex crimes. Its duration is dependent both on the victim's metabolism and the 'strength' of their powers, and there have been issues with unintended fatal overdoses. Its side effects can include disorientation, drowsiness, and temporary memory loss. It is also technically available in ways that would allow for recreational use, but is really much more commonly used to facilitate crimes.
Both drugs can be administered either orally or via injection, with the injectable option providing a much faster result.