(Reuters) - The U.S. Food and Drug Administration issued warning letters on Tuesday to two privately held companies for illegally selling unapproved, misbranded drugs containing kratom claiming to cure opioid addiction and withdrawal symptoms.

Leaves of the kratom tree, native to southeast Asia, can be used as a stimulant or sedative and the U.S. Drug Enforcement Administration has listed it as a “drug and chemical of concern”.

The FDA warning letters were issued to Cali Botanicals and Kratom NC. Both the companies did not immediately respond to requests for comment.

Advocates say kratom helps ease pain and reduce symptoms of opioid withdrawal, but the FDA has not approved it for any medical use.

The FDA has been clamping down on the substance and has in the past issued warnings to companies marketing these products for opioid use disorder. The U.S. health regulator had also warned of traces of high levels of heavy metals like lead in some kratom products.

"As we work to combat the opioid crisis, we cannot allow unscrupulous vendors to take advantage of consumers by selling products with unsubstantiated claims," Acting FDA Commissioner Ned Sharpless said on Tuesday. (reut.rs/2NaJPBT)