As happens from time to time, a new drug has come onto the global market. Like many so-called miracle drugs, its purpose was to essentially turn men into even manlier men. Nothing especially new. Nobody expected amazing results, and many assumed it would quickly fade back into obscurity and gas station advertisements.
Upon testing however, it was discovered that this drug had a unique chemical composition (mainly consisting of heavy myostatin inhibitors, massive amounts of testosterone, and several other compounds which would probably alert the FBI if they were to be written) which made it extremely fast-acting and incredibly effective. Test subjects found themselves taller, more muscular, occasionally hairier, and usually sporting a greatly enlarged... appendage, often within minutes to hours. Extremely heightened, long-lasting arousal was also a common side effect, sometimes resulting in homosexual behavior in heterosexual subjects.
As a result of these rapid and sometimes unpredictable effects, as well as the discovery that the drug was undetectable when dissolved in liquids such as water, it was rapidly pulled from public use and knowledge and remains heavily guarded by governments and research institutes. However, that doesn't mean it won't sometimes end up where it shouldn't be....