More than a year after backtracking on plans to ram through a ban on the herbal supplement kratom, federal authorities once again appear to be getting ready to crack down on the botanical drug used by many Americans.

On Tuesday, Scott Gottlieb, commissioner of the Food and Drug Administration, issued a public health advisory on kratom, outlining concerns about its “deadly risks.”

Kratom, which is derived from the leaves of a Southeast Asian tree related to coffee, has been consumed for millennia, usually as a tea or powder. The product sold in the U.S. today ranges from bags of raw kratom powder, marketed as an herbal supplement, to consumables in brightly colored packaging, found at gas stations and pushed as a cheap high, with minimal information about what might be mixed in with the kratom.

Gottlieb’s statement outlined concerns about the herb’s potential for abuse and addiction. He claimed there was “clear data” on these harms, pointing to reports of 36 deaths associated with kratom-containing products and a tenfold increase in calls involving kratom to U.S. poison control centers from 2010 to 2015.

Although kratom users have touted the herb as a safe and effective treatment for conditions like chronic pain, anxiety and depression, as well as a replacement drug for opioid addiction, Gottlieb said there is “no reliable evidence to support” those applications. In particular, he called the use of kratom as an opioid substitute “extremely concerning” and suggested that it could actually “expand the opioid epidemic.”

While kratom is not an opioid, its active ingredients mimic some opioid effects, including euphoria.

“We’ve learned a tragic lesson from the opioid crisis: that we must pay early attention to the potential for new products to cause addiction and we must take strong, decisive measures to intervene,” said Gottlieb. “From the outset, the FDA must use its authority to protect the public from addictive substances like kratom, both as part of our commitment to stemming the opioid epidemic and preventing another from taking hold.”

In a speech later on Tuesday, Gottlieb said the FDA would use “import alerts and other authorities” to increase seizures of kratom coming into the U.S. He also appeared to lump kratom in with a broader category of “illegal opioids,” of which he said 340 million packages enter the U.S. each year.

The FDA previously issued import alerts in 2012 and 2014, reportedly leading to the confiscation of thousands of pounds of kratom products.

Alissa Scheller An assortment of kratom products.

Gottlieb’s characterization of kratom stands in stark contrast to the one coming from its advocates, who last year fended off a Drug Enforcement Administration push to place kratom’s active ingredients on the list of Schedule I drugs. (Schedule I, which includes heroin and LSD, is supposed to identify the most dangerous substances, with a high potential for abuse and no known medical benefit.) Many kratom users claim it’s nothing short of a miracle drug, which has allowed some people to manage chronic pain and even mental health problems without the use of more powerful pharmaceuticals.

Apart from these glowing anecdotes, there’s also a growing body of scientific evidence supporting kratom’s therapeutic value ― or at the very least, suggesting that it is “much less harmful than prescription opioids.” Kratom contains an alkaloid called mitragynine, which appears to activate opioid receptors in the brain and reduce pain. Although most opioids have sedative qualities, low to moderate doses of kratom serve as a mild stimulant.

Last year, researchers at Columbia University published a study on mitragynine finding that it activates opioid receptors without triggering respiratory depression, the side effect of narcotic painkillers that is typically responsible for fatal overdoses. The scientists concluded that drugmakers could look to kratom to develop safer pain medications. At least one pharmaceutical company is currently working to develop a medication that targets pain with a semi-synthetic compound derived from kratom.

Despite those promising developments, the FDA has not yet approved kratom itself for any medical purpose. Gottlieb noted that fact in his statement, saying that kratom vendors had not gone through the FDA process for evaluating botanical drug products. He encouraged researchers to continue conducting “research that will help us better understand kratom’s risk and benefit profile.”

Gottlieb’s statement underscores the tricky regulatory challenge that kratom faces. Developing a drug that will win FDA approval can cost more than $2 billion. Since the product in this case is a leaf that’s been around for thousands of years and cannot be patented on its own, Big Pharma has little financial incentive to go down that road.

In the meantime, the federal targeting of kratom could lead to additional problems for legitimate users of the herb, as well as others who might benefit from taking it, said Jag Davies, director of communications strategy at the Drug Policy Alliance.

“It’s outrageous that this is happening at a time when we really need innovative solutions to deal with overdose and addiction right now,” he told HuffPost. “We know that kratom is a lot less harmful than lots of these other drugs. They’re basically pushing people into using more dangerous substances.”

Although the FDA’s statement does nothing to officially change kratom’s legal status, it suggests that the federal government could once again be thinking about prohibition. Last October, the DEA postponed its kratom ban pending a public comment period. More than 99 percent of those submissions opposed putting kratom’s ingredients in Schedule I, according to an analysis by the American Kratom Association, a nonprofit trade group and the unofficial lobbying arm of kratom advocates.

At the same time, the DEA asked the FDA to conduct a formal scientific and medical evaluation of kratom to help guide any future decision. That analysis is ongoing, Gottlieb said Tuesday.

In recent years, a number of states have also passed legislation to ban the herb, often on the backs of reports about fatal incidents involving kratom.

The American Kratom Association has routinely pushed back against such reports, arguing that the deaths had other contributing factors. Last year, the DEA said there had been a total of 30 documented deaths associated with kratom around the world. Toxicology tests found that many subjects were abusing other drugs or suffered from pre-existing health conditions when they died.

The FDA did not immediately release additional information to HuffPost on the 36 kratom deaths that Gottlieb cited on Tuesday. Andrew Kruegel, a Columbia University chemist who was involved in the mitragynine study, was skeptical of the number, but wouldn’t rule out the potential of harmful interactions involving raw kratom powder.

“What we research is primarily pure isolated compounds and their effects, but what people are taking is this plant that has variable composition,” he said. “I’m pretty comfortable saying that mitragynine itself is a promising lead for new analgesics, but I’m not sure what I can say about raw kratom powder. A logical extension is that it would also have promising properties and would probably be even safer than the pure compounds because it’s a lower concentration, but you never know what else is in the plant.”

That said, Kruegel thinks the entire debate over the proper policy response to kratom has suffered from a lack of scientific evidence.

“It’s so hard to evaluate the risk-benefit ratio when the risks are not well-defined and the benefits are not well-defined. We don’t have clinical trials to show that it has therapeutic properties, but we also don’t have rigorous epidemiology to show the extent of abuse or the extent of toxicological incidents from kratom consumption,” he said.

In short, Kruegel said nobody can do a fair analysis of how kratom should be regulated right now.

UPDATE: In a statement Wednesday, the American Kratom Association called on the FDA to “rescind” its public health advisory, arguing that the agency had relied upon “discredited, incomplete, and mischaracterized scientific claims” about the herb.

“For years, the FDA has published scientifically inaccurate information on the health effects of consuming kratom, directly influencing regulatory actions by the DEA, states, and various local government entities,” the association said. “It is inconceivable that the FDA would favor a policy that would foreseeably force a patient who [has] been weaned off of opioid [addiction] back to dangerously addictive and potentially deadly opioid prescription medications.”

The American Kratom Association added that it plans to submit a formal dispute resolution petition with the U.S. Department of Health and Human Services to challenge the FDA’s advisory.