The U.S. Food and Drug Administration said on Tuesday it had found high levels of heavy metals such as lead and nickel in some kratom products, following field investigations.

The regulator has been clamping down on the substance, which advocates say help ease pain and reduce symptoms of opioid withdrawal, but which the FDA says has similar effects to narcotics such as opioids and has resulted in dozens of deaths.

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Levels of metal in kratom-based products would likely not result in poisoning based on single use, but could cause people to suffer if used persistently, FDA commissioner Scott Gottlieb said.

"The findings of identifying heavy metals in kratom only strengthen our public health warnings around this substance," Gottlieb said here in a statement.

While kratom is not controlled under the Federal Controlled Substances Act, the U.S. Drug Enforcement Administration has listed it as a “drug and chemical of concern”.

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Leaves of the kratom tree, native to Southeast Asia, can be used as a stimulant or sedative.

The FDA said it was contacting the companies marketing kratom products to inform them of the metal testing results.

The agency has in the past issued warnings against companies marketing these products for opioid use disorder, and has warned of high rates of salmonella in the products.