The Drug Enforcement Agency (DEA) responded to rising concern from both the medical and law enforcement communities over acetyl fentanyl by temporarily reassigning the substance as a Schedule I drug, per the provisions of the Controlled Substances Act.

The compound, which was first identified by a government laboratory in Maine in 2013, is related to the prescription painkiller fentanyl and a small amount can produce a euphoric response in users that resembles a high from heroin or oxycodone, which led to it being dubbed “fake heroin.”

As a result, the drug has been either falsely sold as both of those substances, or mixed with either substance to produce a stronger high. The danger of such a combination is due to acetyl fentanyl’s extreme potency—it’s described as five times more powerful than heroin. It has resulted in a wave of emergency room admissions throughout the United States, and a connection to at least 39 overdose deaths.

The drug is so strong that first responders and medical professionals must administer larger and even double the amount of doses of the opioid antagonist naloxone in order to counteract an overdose.

Further complicating attempts to gain control of acetyl fentanyl’s rise in the drug market is its vague legality. Though considered illicit for human use, it went unregulated and was actually legal to buy if it bore a label marked “not for human consumption.”

The DEA’s response, which was described in their press release as “necessary to avoid an imminent hazard to the public safety,” removes the loophole that allowed distributors to sell the drug without interference, and imposes the same degree of civil and criminal penalties that would be generated by the sale, possession and distribution of heroin, LSD, GHB, mescaline, and marijuana in states where it remains illegal for use.