SHANGHAI — So deadly it’s considered a terrorist threat, carfentanil has been legal in China— until now. Beijing is banning carfentanil and three similar drugs as of March 1, China’s Ministry of Public Security said Thursday, closing a major regulatory loophole in the fight to end America’s opioid epidemic.

“It shows China’s attitude as a responsible big country,” Yu Haibin, the director of the Office of the National Narcotics Control Committee, told the Associated Press. “It will be a strong deterrent.”

He added that China is actively considering other substances for sanction, including U-47700, an opioid marketed as an alternative to banned fentanyls. China said the March 1 ban will also apply to carfentanil’s less-potent cousins furanyl fentanyl, acryl fentanyl and valeryl fentanyl.

The U.S. Drug Enforcement Administration called China’s move a potential “game-changer” that is likely to have a big impact in the U.S., where opioid demand has driven the proliferation of a new class of deadly drugs made by nimble chemists to stay one step ahead of new rules like this one. After China controlled 116 synthetic drugs in October 2015, seizures in the United States of compounds on that list plunged.

“It’s a substantial step in the fight against opioids here in the United States,” said Russell Baer, a DEA special agent in Washington. “We’re persuaded it will have a definite impact.”

Legally used as an anesthetic for elephants and other large animals, carfentanil burst into the North American drug supply last summer, causing hundreds of unsuspecting drug users to overdose. The DEA confirmed more than 400 seizures of carfentanil across eight U.S. states from July through October. So lethal an amount smaller than a poppy seed can kill a person, carfentanil was researched for years as a chemical weapon and used by Russian forces to subdue Chechen separatists at a Moscow theater in 2002.

New data from DEA laboratories suggests the supply of furanyl fentanyl is now surging. DEA labs identified 44 samples of furanyl fentanyl in the last three months of 2016, up three-fold from the prior quarter.

Though Beijing has said U.S. assertions that China is the top source of fentanyls lack evidence, the two countries have deepened cooperation as the U.S. opioid epidemic intensifies. Beijing already regulates fentanyl and 18 related compounds, even though they are not widely abused domestically. Since 2016, China has arrested dozens of synthetic drug exporters, destroyed eight illegal labs and seized around 2 tons of new psychoactive substances, according to the Office of the National Narcotics Control Committee.

But the battle against rapidly evolving synthetic drugs is complicated by the deeply global nature of the narcotics trade and the deeply national nature of law enforcement. Some online drug vendors host their websites on servers abroad to thwart police. All benefit by submerging their illicit packages in the vast tides of legitimate commerce shipped or sent by courier from China.

One example of the kind of global coordination needed to take down synthetic drug barons is the case of Zhang Lei, whom the U.S. Treasury Department designated a drug kingpin in 2014. China shared 4,221 clues with 58 countries and areas in the hunt for Zhang, Chinese drug control authorities said Thursday. Zhang was sentenced to 14 years in Chinese prison last year, according to one of his lawyers, Fan Renzhong.

In October, the AP identified 12 Chinese companies willing to export carfentanil around the world for a few thousand dollars a kilogram (2.2. pounds), no questions asked. That same month China began evaluating whether to ban carfentanil and the three other drugs. Usually, the process can take nine months. This time, it took just four. Good international cooperation and effective early warning systems to track the emergence of novel drugs helped speed the process, Chinese drug control officials said Thursday.

China’s action is “a hopeful sign of political and strategic law enforcement cooperation,” said Jeremy Douglas, a regional representative for the U.N. Office on Drugs and Crime in Bangkok. “But having legislation is a first step. The law will need to be enforced effectively.”

Offers for carfentanil from Chinese vendors were scarce Thursday, but the AP quickly secured five offers to export furanyl fentanyl to the United States. Some vendors also pushed U-47700.

“One news I just got is that the carfentanil and furanyl fentanyl etc opioid analogs will be controlled in China on March 1 effective,” one vendor called Ete wrote in an email. “So if you need them pls make it before that day. After that day it will be unavailable.”

The vendor did not immediately respond to a request for comment from AP.