Kratom, a Southeast Asian plant known for its opiate-like effects, may put users at risk of addiction and even death, the US Food and Drug Administration said in a public health advisory today. The drug has been rising in popularity as an opioid alternative.

Kratom, which has been consumed in Southeast Asia for centuries, binds to the same opioid receptor as morphine — so it can treat pain. Some chemists figure it’s a promising starting point for new, less addictive painkillers. But it’s currently taken as a recreational drug, with its users saying it treats anxiety, depression and opioid withdrawal. These anecdotal reports aren’t the same as real medical research — and more is needed to determine the plant’s medical benefits. “At a time when we have hit a critical point in the opioid epidemic,” the FDA statement said, “the increasing use of kratom as an alternative or adjunct to opioid use is extremely concerning.”

“extremely concerning.”

Last year, the US Drug Enforcement Administration announced its plans to name the substance a Schedule I drug, the most restrictive classification alongside heroin and LSD which restricts medical research. After a public outcry, the DEA decided to postpone its decision, and asked the FDA to weigh in.

The potential safety risks for kratom raise “significant concerns,” according to the FDA. The FDA is aware of 36 deaths linked to the use of products containing kratom. Calls to poison control centers regarding kratom have increased 10-fold from 2010 to 2015.

For now, kratom is widely available online and in some stores. But it may be laced with other substances, such as opioids like hydrocodone, the FDA says. Taking kratom is also linked to serious side effects like seizures, liver damage, and withdrawal symptoms. People who are taking the product to treat depression and opioid addiction may be putting themselves at risk, the agency says. “Patients addicted to opioids are using kratom without dependable instructions for use and more importantly, without consultation with a licensed health care provider about the product’s dangers, potential side effects or interactions with other drugs,” FDA Commissioner Scott Gottlieb wrote in the advisory.

If companies want to market kratom for its supposed health benefits, they should go through the FDA’s drug review process, Gottlieb said. That way the product can be assessed for its safety and effectiveness. But for now, no company has tried to properly develop a drug that includes kratom.

“those uses must be backed by sound science.”

“While we remain open to the potential medicinal uses of kratom, those uses must be backed by sound science and weighed appropriately against the potential for abuse,” Gottlieb wrote. Until someone does the science the agency is requesting, the FDA will attempt to prevent shipments of kratom from entering the US. The agency has already detained hundreds of packages at international mail facilities.

Whether kratom will eventually be banned, however, depends on the Drug Enforcement Administration. The DEA will review the FDA’s assessment and make a determination, says DEA spokesperson Wade Sparks. “That would be the next step and that has always been historical the process,” Sparks tells The Verge.