A drug so deadly it’s considered a terrorist threat, carfentanil has been legal in China up to now — but no longer. From March 1, it and three similar drugs will be banned, the Ministry of Public Security announced yesterday.

“It shows China’s attitude as a responsible big country,” Yu Haibin, director of the Office of the National Narcotics Control Committee, told reporters.

The ban will also apply to carfentanil’s less-potent cousins furanyl fentanyl, acryl fentanyl and valeryl fentanyl.

Yu said China is also actively considering other substances for sanction, including opioid U-47700, which is marketed as an alternative to fentanyls.

The US Drug Enforcement Administration called China’s move a potential “game-changer” that is likely to have a big impact in the US, where opioid demand has driven the proliferation of a new class of deadly drugs made by nimble chemists staying one step ahead of new rules like this one.

“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 users to overdose. The DEA confirmed more than 400 seizures of the drug across eight states between July and October.

So lethal that an amount smaller than a poppy seed can kill, 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 in the last three months of 2016, up three-fold from the prior quarter.

Though Beijing has said US assertions that China is the top source of fentanyls lack evidence, the two countries have deepened cooperation as the US opioid epidemic intensifies. China 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.

One example of the global coordination needed to take down synthetic drug barons is the case of Zhang Lei, designated as a drug kingpin by the US Treasury Department in 2014. China shared 4,221 clues with 58 countries and areas in the hunt for Zhang, its drug control authorities said yesterday.

Zhang was sentenced to 14 years in a Chinese prison last year, according to one of his lawyers, Fan Renzhong.

In October, The Associated Press identified 12 Chinese companies willing to export carfentanil around the world, 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.