Remember the guy who bought 80-proof vodka that turned out to be 190-proof Everclear and died from alcohol poisoning? Probably not, because that sort of thing almost never happens in a legal market, where merchants or manufacturers who made such a substitution, deliberately or accidentally, would face potentially ruinous economic and legal consequences. In a black market, by contrast, customers frequently get something different from what they thought they were buying: something weaker, something stronger, or some other substance entirely.
As The Washington Post noted in September, the results can be fatal. Fentanyl, a synthetic painkiller, is around 40 times as strong as pure heroin. Heroin dealers therefore have been known to mix fentanyl into their product, giving it an extra kick that partly makes up for the dilution that occurs between production and retail sale. The Post cited three fatal overdoses involving fentanyl-spiked heroin in New York and Connecticut, plus other cases where users "had to be resuscitated at hospitals."
Although such fatalities are commonly called "drug-related deaths," they are more appropriately viewed as prohibition-related deaths. The artificially high prices and profits created by prohibition give dealers a strong incentive to dilute their products, and the black market's lack of legal accountability allows them to do so. If they go too far and customers start to balk, adding a little fentanyl is a cheap, easy solution.
Not only does prohibition create the hazard of an unpredictable product, but enforcing prohibition, to the extent that it has any effect at all, exacerbates the problem. Drug warriors commonly cite lower potency as a sign their efforts are succeeding. When heroin gets weaker, users learn to take larger doses—a habit that can be deadly when they encounter an unusually strong or fentanyl-spiked batch.