A group of scientists is challenging an FDA report released this week warning against the use of kratom, based in part on an FDA analysis that emphasized the substance’s similarity to traditional opioids like morphine and heroin.

“Taken in total, the scientific evidence we’ve evaluated about kratom provides a clear picture of the biologic effect of this substance,” FDA Commissioner Scott Gottlieb said in a statement. “Kratom should not be used to treat medical conditions, nor should it be used as an alternative to prescription opioids. There is no evidence to indicate that kratom is safe or effective for any medical use.”

FDA officials have recommended kratom be categorized as a Schedule 1 drug, which would effectively mean that scientists would not be able to study its effects. Advocates of kratom have long said that the plant-derived substance can be safer than opioid drugs sold at a pharmacy or on the street, fighting pain and treating symptoms of opioid withdrawal without the same, potentially deadly effects on respiration.

Federal officials have occasionally seized imported shipments of kratom on behalf of the FDA, saying it’s a new, untested dietary ingredient. But it’s still widely available, and in most states is readily sold as a dietary supplement in convenience stores, head shops, kratom specialty stores, and online.

“The available science is clear that kratom, although having effects on opioid receptors in the brain, is distinct from classical opioids (e.g. morphine, heroin, oxycodone, etc.) in its chemistry, biological effects, and origin (kratom is a tree in the coffee family, not the opium poppy family),” the scientists wrote, in an open latter to Gottlieb and White House advisor Kellyanne Conway, and circulated by the American Kratom Association, an industry group.

“Importantly, as commonly used in raw plant form, it does not appear to produce the highly addictive euphoria or lethal respiratory depressing effects of classical opioids,” the group, including scientists from Johns Hopkins University, Columbia University and the University of Rochester, wrote. Conway has been overseeing White House efforts to combat the national opioid crisis.

Questions About The FDA’s “Novel” Analysis, Death Estimate

Scientists have known since the ’90s that kratom contains alkaloid compounds that exhibit mild opioid activity. That made the FDA’s warning puzzling–and frustrating–for some researchers.