You know a drug is bad when drug dealers won’t even supply it.

That’s the case with fentanyl, a powerful synthetic opioid, which is now being delisted by major dark web drug suppliers because of the risks associated with it. Because fentanyl is so deadly, it’s more likely to trigger investigation from authorities. To keep the heat off, illegal online marketplaces are beginning to voluntarily ban the substance, particularly in the United Kingdom.

“If they’ve got people selling very high-risk commodities then it’s going to increase the risk to them,” said Vince O’Brien of the U.K.’s National Crime Agency. “There are marketplaces that will not accept listings for weapons and explosives — those are the ones that will not accept listings for fentanyl.”

Fatalities associated with fentanyl in the U.K. have risen by 30% in the past year, with a total of 160 deaths since the drug hit the scene in 2017. In the U.S., an estimated 29,000 people were killed by fentanyl, either on its own or mixed with another drug, in 2017.

The potent substance, medically used as an anesthetic, is often cut into heroin to make a dealer’s supply last longer or become more effective. Fentanyl is 100 times stronger than heroin, making accidental overdoses easy when drug users believe they are only taking heroin.

While the bans were likely motivated by self-preservation, dark net operators are certainly aware of the effects of fentanyl on the public.

“There are also drug users on the dark web who say on forums that they don’t think it’s right that people are selling fentanyl because it is dangerous and kills a lot of people,” O’Brien said.

Tox Tuesday is our regular series on the latest toxicology news, from opioid crisis updates to developing testing technology and more. Check it out here.