Prohibition killed Philip Seymour Hoffman
According to the Substance Abuse and Mental Health Services Administration, fentanyl is "roughly 50-80 times more potent than morphine," so it's the sort of ingredient you'd want to know about before snorting or injecting that white powder you just bought. This kind of thing—passing one drug off as another, delivering something much more (or less) potent than the customer expects—almost never happens in a legal market. When was the last time you bought a bottle of 80-proof whiskey that turned out to be 160 proof? The main reason liquor buyers do not have to worry about such a switcheroo is not that distillers are regulated, or even that their customers, unlike consumers in a black market, have legal recourse in case of fraud. The main reason is that legitimate businesses need to worry about their reputations if they want to keep customers coming back. It is hard to build and maintain a reputation in a black market, where brands do not mean much.