Just two salt-sized specs of it can kill.
It's 10,000 times more potent than morphine, and 2 milligrams of it can stop a one-ton elephant in its tracks.
Now, carfentanil is being blamed for five overdose deaths in Minnesota.
This synthetic opiate was never meant for humans — because of its lethality, Russian agents have reportedly used it in assassination attempts.
Still, carfentanil has found its way onto the streets of Minnesota and other states — via the dark web, a largely hidden region of the internet that can require special software or authorization to access.
Black market economics have changed a lot in past few years, said Dr. Joseph Lee, medical director of the Hazelden Betty Ford Foundation Youth Continuum.
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Bitcoin — an anonymous, encrypted digital currency — is a major catalyst in bringing carfentanil into the mainstream drug trade. It often comes from overseas into the hands of American drug dealers.
Carfentanil labs have popped up across the world, many in China. They ship their product in the mail to people who cut it into other drugs, often heroin, and deal them.
When Bitcoin became popular, Lee said, it became easier for people to trade illicit drugs like carfentanil.
Labs would set up stores on the dark web, providing some sophistication and security for buyers on the black market.
"These shops online in the dark web started to become like Amazon," Lee said. "There are reviews. And that consumer, detailed review gives people confidence in what they're getting."
Dealers who buy carfentanil cut it into heroin and other drugs to boost profit margins. It's relatively cheap and super potent, making it enticing for some dealers.
But the dealers don't often know what they're doing when mixing drugs. Accidents happen, and people die.
Democratic Sen. Amy Klobuchar is pushing a bill that aims to stop synthetic drugs from making their way to the U.S. by requiring foreign shipments to come with data that would help customs agents better identify illegal packages.
Lee said that would be helpful, but only a start at ending the problem.
"This route of getting drugs onto our streets is probably not going to go away quickly, unfortunately," he said. "Even if we shut down labs in China, there will be other secretive labs in different places, maybe even here in the United States."