A new study from the Centers for Disease Control and Prevention (CDC) found that kratom was a cause of death in nearly 100 people over a 17-month period -- a higher number than previously reported.
The CDC analyzed reports of more than 27,000 unintentional drug overdoses across 32 states and Washington, D.C.
The study said 152 of those people were found to have kratom in their system, and the drug was a cause of death for 91 of them.
Earlier this year, the US Food and Drug Administration said that 44 deaths had been associated with the drug.
About 80% of the people in the CDC study who had kratom in their system had a history of substance misuse. Almost all of them also had other substances in their system.
The other substances most commonly found were fentanyl, heroin, benzodiazepines (also known as tranquilizers), prescription opioids and cocaine.
The number of people whose deaths were associated with kratom make up less than 1% of all overdose deaths. The report's authors say it's possible the numbers might be underestimated, although they can't say to what extent.
Health officials have warned about kratom
Nearly five million Americans consume kratom safely each year, according to the American Kratom Association.
The Southeast Asian plant is sold in pills, capsules and extracts. Some people brew the leaves as a tea, while others chew or smoke them.
At low doses, kratom acts like a stimulant. At higher doses, it sedates dulling pain -- which has made it a popular over-the-counter remedy for opioid withdrawal.
But health officials have urged caution.
FDA Commissioner Scott Gottlieb likened the chemical compounds in kratom to opioids and said there was no evidence to indicate that it was safe or effective for "any medical use." He also cited concerns about its potential for abuse.
"Kratom should not be used to treat medical conditions, nor should it be used as an alternative to prescription opioids," Gottlieb said in a statement in 2018.
The Drug Enforcement Administration lists kratom as a drug of concern but dropped a plan to classify it as a controlled substance after public outcry in 2016.
US poison control centers saw the number of calls about kratom exposures increase from 13 in 2011 to 682 in 2017, often for serious unanticipated effects of the supplement, according to a study published earlier this year.
"Individuals who choose to use kratom need to be aware of the potential risks. Just because it is currently classified as an herbal supplement does not mean that it is regulated or that it is safe," Henry Spiller, director of the Central Ohio Poison Center at Nationwide Children's Hospital, said in a statement in February.
Researchers question the FDA's warnings
Some scientists who study kratom disagree with the FDA's 2018 warning about the drug's potential for abuse.
Scott Hemby, who chairs the Department of Basic Pharmaceutical Sciences at High Point University in North Carolina, researches "addictiveness" of kratom and spoke to CNN after the FDA issued its warning.
He found that two chemicals in kratom, mitragynine and 7-hydroxymitragynine, can bond to the body's opioid receptors and act very much like opioids do, providing some pain relief and causing a release of dopamine.
But it's at a much more toned-down level than prescription pills or heroin.
Hemby did find that 7-hydroxymitragynine may have some addictive qualities, but when looking at kratom as a whole, the ratio of that element is so small that the drug has very low abuse potential, he told CNN.