Federal authorities on Friday announced the seizure of what they consider to be a toxic botanical substance from a central California firm that has been selling it as a natural medicine.

U.S. Marshals seized more than 100 cases of health products containing the unapproved ingredient Kratom from a Grover Beach-based distributor, Nature Therapeutics, which does business as Kratom Therapy, according to the U.S. Attorney's Office for the Central District of California and the Food and Drug Administration.

The substance comes from the leaves of the tropical Kratom tree, a Southeast Asian evergreen that's part of the coffee family. It has long been used as an herbal medicine in Southeast Asia, where people typically chew the leaves. But it can also be ingested in powdered or liquid form, says FDA spokeswoman Lyndsay Meyer.

Kratom is legal under federal and California law, but the FDA has not approved it as medicine.

Consumers use Kratom to treat a variety of ailments, including diarrhea, pain and addiction.

At low doses it works as a stimulant, and at higher doses it has the opposite effect, says Dr. Michael Levine, an assistant professor of emergency medicine and toxicology expert at Keck Medicine of USC.

"Someone has a little bit of Kratom, they get a euphoric and kind of high sensation and a little bit stimulated," Levine says. "If they take a lot of it, they get sedated effects."

In the U.S., Kratom is becoming more popular as an organic substitute for addictive opioid painkillers.

But the National Institute on Drug Abuse website states: "There is no scientific evidence that kratom is effective or safe for this purpose."

And, says Levine, "there is certainly some addiction potential with Kratom. Just because it's available online doesn't mean it's safe."

Kratom has some staunch defenders. The American Kratom Association says on its website that it was founded in 2014 in part to combat "misinformation, both scientific and anecdotal," and to lobby lawmakers and regulators.

The Association claims Kratom "has been used for hundreds of years to safely alleviate pain, combat fatigue and help with the effects of anxiety and depression."

Citing the FDA's concerns about Kratom, the U.S. Attorney's office said in a statement Friday that consumption "can lead to a number of health impacts including, respiratory depression, vomiting, nervousness, weight loss and constipation. Kratom has been indicated to have both narcotic and stimulant-like effects and withdrawal symptoms may include hostility, aggression, excessive tearing, aching of muscles and bones and jerky limb movements."

The seized products are worth about $150,000, according to the U.S. Attorney's office.