You might think the correct answer is “fentanyl,” as this piece in the New York Times suggests. Fentanyl is a synthetic opioid roughly 50 times stronger than heroin.
That answer, however, is incomplete.
For legal goods, more and less potent versions co-exist, and consumers choose the potency level that suits their preferences (e.g., wine coolers versus whiskey).
Likewise, for legal goods, expensive and inexpensive versions co-exist, but at different prices (e.g., Two Buck Chuck versus Chateau Blah-Blah-Blah 1886).
And, for legal goods, people rarely overdose on even the most potent versions, because they have accurate information on potency.
Heroin’s disappearance, therefore, results from prohibition. Dealers prefer fentanyl because it is compact and easy to hide from law enforcement. Dealers specialize in heroin because, in contrast to legal merchants, they cannot easily display a range of products and let consumers choose.
And the high overdose rate for fentanyl results from prohibition; were it legal, consumers would know how much fentanyl to consume and have ready access to Narcan (which reverses the life-threatening effects of opioids).
All opioids can cause overdoses if used in excess (like many legal products).
But opioids can also be used indefinitely if consumed in an appropriate dosage.
Prohibition, not fentanyl, is the real killer.