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The Food and Drug Administration declared the popular herbal product kratom to be an opioid on Tuesday, opening a new front in its battle to get people to stop using it.

New research shows kratom acts in the brain just as opioids do, FDA Commissioner Dr. Scott Gottlieb said in a statement. And he said the agency has documented 44 cases in which kratom at least helped kill people — often otherwise healthy young people.

“Taken in total, the scientific evidence we’ve evaluated about kratom provides a clear picture of the biologic effect of this substance,” Gottlieb wrote.

Kratom is used by some as a home remedy for opioid addiction — and by others just for fun. It has a passionate following.

“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.”

Last November, the FDA cautioned people not to use kratom.

Supporters of kratom use have been fighting to keep it legal for years. The Drug Enforcement Administration temporarily listed kratom as a Schedule 1 controlled substance last August, but withdrew the decision after an outcry and a targeted petition effort.

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Related: Kratom can kill you, FDA says

The FDA says scientific evaluation shows there is no wiggle room.

“As the scientific data and adverse event reports have clearly revealed, compounds in kratom make it so it isn’t just a plant — it’s an opioid,” Gottlieb wrote.

Sellers market it as a safe, "plant-based" product. But Gottlieb pointed out that heroin comes from plants, also.

There are drugs on the market that have been demonstrated to be safe and effective for helping people treat opioid addiction, he said.

“For individuals seeking treatment for opioid addiction who are being told that kratom can be an effective treatment, I urge you to seek help from a health care provider,” Gottlieb said.

Related: Stop saying cannabis cures cancer, FDA warns

“Combined with psychosocial support, these treatments are effective. Importantly, there are three drugs (buprenorphine, methadone, and naltrexone) approved by the FDA for the treatment of opioid addiction, and the agency is committed to promoting more widespread innovation and access to these treatments to help those suffering from an opioid use disorder transition to lives of sobriety,” he added.

The American Kratom Association said the FDA is biased against the product.

"This is an unprecedented abuse of science to create a new computer program that is clearly garbage in garbage out avoiding the rules of the Controlled Substances Act and making unproven claims that have been proven to be untrue," the AKA said in a statement.

However, people who sell kratom have not conducted the safety and efficacy tests needed to sell a product that carries medical claims, the FDA points out. It takes years of trials to show whether a product works as people believe it does.

Gottlieb noted that some people also use kratom to try to treat pain.

“There are also safer, non-opioid options to treat pain," he wrote.

"We recognize that some patients have tried available therapies, and still have unmet medical needs. We’re deeply committed to these patients, and to advancing new, safe and effective options for those suffering from these conditions.”

The FDA released detailed accounts of several of the deaths. The victims often had mixed kratom with other substances, including chemicals taken out of inhalers and found in over-the-counter cold and flu drugs.

Kratom is widely available online. Federal agents have seized some kratom from retailers, but it was not immediately clear what the federal government plans to do about the many online outlets.