The Drug Enforcement Administration declared on Friday that it would temporarily reassign acetyl fentanyl — commonly known as "fake heroin" — as a Schedule I drug, a designation reserved for the most dangerous substances.

In a statement, the DEA said the move was "necessary to avoid an imminent hazard to the public safety." The drug, which is up to 15 times stronger than heroin, has been blamed for at least 39 overdose deaths since it was first identified by a government laboratory two years ago. But, the DEA says, reports of emergency room admissions due to overdoses are probably underreported because it's so hard to differentiate from heroin.

Before the DEA's declaration, the drug existed in something of a legal grey area: It was considered illicit, but it was still technically legal as long as it bore a label that read, "Not for human consumption." By rescheduling it, the government has changed that.

According to the DEA, chemically similar drugs emerged on the West Coast where they were "trafficked and abused" in the late 1970s and 1980s. The acetyl fentanyl known today was first identified in Maine in 2013. Reports of the drug nearly quadrupled the following year, as it popped up in states from Louisiana to North Dakota to Washington.

In August of last year, the American College of Emergency Physicians warned ER doctors to prepare for an upswing in cases that might appear at first to be heroin overdoses. Acetyl fentanyl is so strong it requires a larger, or even double, dose of Naloxone — known under the brand name Narcan — the drug used to counteract overdoses.

Since it is so much more potent and doses are smaller, the difference between a therapeutic (or recreational) dose of acetyl fentanyl and a lethal dose is much smaller than heroin.

Authorities have busted at least one acetyl fentanyl supplier, based in Montreal, since the drug was identified. Three kilograms were confiscated in that raid, but, the DEA said on Friday, "Given that a typical dose of acetyl fentanyl is in the microgram range, a three-kilogram quantity could potentially produce millions of dosage units."

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