The FDA is taking public comments ahead of a United Nations meeting that plans to add 10 drugs to international lists of controlled substances, including several relatives of powerful street drugs.

Two synthetic opioids are being proposed for the most restricted class of controlled substances: U-47700, which was developed in the 1970s and has been placed in Schedule I in the U.S. by the Drug Enforcement Administration; and butyrfentanyl, an analogue of fentanyl, with an abuse profile similar to other opioid agonists. Unlike fentanyl, butyrfentanyl has no approved medical use in the U.S.

In the next class of restricted substances, the World Health Organization proposed 4-MEC, 3-MMC, pentedrone, and ethylone — all synthetic drugs similar to amphetamine — as well as methiopropamine, an analogue of methamphetamine.