Oxycodone. Purdue Pharma sold a formulation of it in their product called OxyContin, which had (still has!) a variety of legitimate medical uses. But it's also

Oxycodone. Purdue Pharma sold a formulation of it in their product called OxyContin, which had (still has!) a variety of legitimate medical uses. But it's also very easy for addicts to abuse, and Purdue pushed doctors to prescribe it widely with few controls, which is generally believed to have sparked the US's huge surge in opioid abuse. (Of course, merely cracking down on oxycodone didn't resolve the problem, because once someone's addicted they become highly motivated to find some other opioid.)