The Food and Drug Administration warned Tuesday that it had found "disturbingly high levels of heavy metals" in kratom products.

FDA scientists said they discovered lead and nickel in 26 products when they were testing kratom for salmonella following an outbreak that had spread across multiple states. The metals were at levels "not considered safe for human consumption," but were not likely to poison someone who had used kratom only once, the FDA said in a statement issued by its commissioner, Scott Gottlieb.

"Some of these products included levels that, with chronic use, could cause some people to suffer from heavy metal poisoning," the FDA said. "We are concerned that there may be other kratom products on the market that also contain heavy metals."

Kratom is an herbal supplement that can be smoked, sipped in tea, or taken as tablets. People who take it have said that it helps relieve chronic pain and depression, and it can work as a replacement for opioids. Some people take it for its stimulating effects.

But the FDA has warned the public against taking kratom, saying its medical benefits are unproven and that it could be linked with 44 deaths since 2011. Critics of the death claims note that the death certificates FDA provided often mention the possible involvement of other drugs.

[Related: Advocates skewer FDA over 'junk science' blaming kratom for deaths]

The latest warning about heavy metals comes after a slew of others earlier this year from the FDA, which has sought to have kratom scheduled by the Drug Enforcement Administration. The agency has recommended categorizing kratom as a Schedule 1 drug, the same designation received by heroin, LSD, and marijuana.

Supporters of keeping the drug legal for research purposes say that such a designation could have a similarly chilling effect on medical research that marijuana has faced. They note federal data show that methadone, a drug often prescribed to treat people with opioid addictions, was responsible for 3,373 deaths in 2016.

The DEA has not said whether it intends to take action. The agency said in October 2016, during the Obama administration, that it would hold off on banning kratom as a Schedule I substance as it waited for additional public comment as well as FDA recommendations.

The latest warning comes as deaths from opioids, including prescription painkillers and heroin, continue to plague communities, claiming more than 40,000 lives in 2017. Gottlieb in his statement honed in on this group, saying he was aware that some of them were taking kratom as a replacement for opioids. He urged them to see a doctor instead.

"We’re deeply committed to these patients, and to advancing new, safe and effective options for those suffering from these conditions," he said. "I assure you that we’ll continue to pursue innovation in these areas."