China's ban on carfentanil may not keep the deadly elephant tranquilizer out of the U.S.

Carfentanil — a synthetic opioid that is roughly 5,000 times stronger than heroin — has long been legal in China, allowing online retailers to sell the drug to customers in the U.S.

But that’s about to change. China’s Ministry of Public Security announced Thursday that it plans to outlaw carfentanil and three other powerful synthetic opioids, effective March 1. The drugs were responsible for a surge in overdoses across the U.S. and Canada in recent months, and the DEA expects the ban to help disrupt an illicit supply chain that stretches from China to North America.

“It’s a huge announcement for us,” DEA spokesman Russell Baer told VICE News. “We’re thinking it’s going to potentially have an immediate and practical impact on drug flows into the U.S.”

But the actual effect of the Chinese ban remains to be seen. Outlaw chemists responded to previous crackdowns on synthetic drugs such as bath salts and Spice by creating new substances that are chemically similar but not yet illegal, staying one step ahead of changes to the law.

“Of course Chinese chemists and global chemists will tweak molecular structures and that sort of thing,” Baer said. “But that’s not going to diminish our aggressive approach to this problem.”

The new Chinese regulations also don’t address the fact that the main chemical precursors used to manufacture carfentanil are still unregulated, which means it will still be relatively easy for drug trafficking organizations in Asia and the Americas to make carfentanil in underground labs.

Baer said the DEA is already talking with China about the next steps.

“I can’t speak on behalf of China,” Baer said, “but we’re hopeful that additional scheduling actions are looming around the corner.”

Legally used by zookeepers as a tranquilizer for elephants and other large animals, carfentanil is about 100 times more powerful than its cousin fentanyl, a common painkiller and surgical anesthetic. The demand for drugs such as carfentanil has surged amid the U.S. opioid crisis, with dealers often cutting the drugs into heroin and causing hundreds of overdoses across the country.

China’s ban also applies to carfentanil’s cousins valeryl fentanyl, acryl fentanyl, and furanyl fentanyl. Baer said furanyl fentanyl is “in top three in terms of national availability of fentanyl compounds,” and that acryl fentanyl has started turning up in seized drugs more often in recent months.

Baer said the DEA had been working closely with Beijing since October to get the drugs outlawed in China.

“The U.S. is the one with the opioid addiction problem,” Baer said. “China, they’ve gone out of their way in many respects to understand our concerns. It’s a global problem, China is a part of hit, as is the U.S., Mexico, and Canada. We’ve been working together to get our arms around it and address some of these issues collectively.”