The herbal supplement kratom has grown in popularity over the past few years, thanks in part to testimonials from people who say it has helped them quit opioids and manage chronic pain. But that popularity has brought negative attention from government authorities. In 2016, the Drug Enforcement Agency considered banning kratom, only to back down under public pressure. The Food and Drug Administration later targeted kratom distributors and released scientifically dubious reports linking kratom to overdose deaths.

Now, the Centers for Disease Control and Prevention has released its own report “to assess the impact of kratom.” The CDC drew on data from 27 states from July 2016 to December 2017—a collection of 27,338 drug-overdose deaths. Of those, 152 were positive for kratom on postmortem toxicology reports.

Which is to say that of nearly 30,000 overdose deaths over an approximately 18-month period, 152 people tested positive for kratom. That’s 0.56 percent, as the CDC report makes clear.

The report fits a familiar pattern, according to Walt Prozialeck, professor and chair of the department of pharmacology at the Chicago College of Osteopathic Medicine at Midwestern University, and an expert on kratom.

“I’m just dumbfounded that they keep doing the same thing,” he says. “Kratom showed up in some of these toxicology reports, and it’s being unofficially blamed as the cause of death, according to this report.”

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He points out that almost all of those 152 who tested positive for kratom also tested positive for other substances; 91 of them (59.9 percent) had kratom listed as a cause of death. Only seven of the 152 tested positive only for kratom, though that doesn’t rule out other unidentified substances.

Those other substances are important, Prozialeck says, because most of them are potentially lethal on their own. Fentanyl and fentanyl analogs, for example, were the most common other substances, appearing as a cause of death in 65.1 percent of the kratom-positive deaths. (Fentanyl is a synthetic opioid said to be more than 100 times more potent than heroin; in 2017, 28,000 deaths involving synthetic opioids occurred in the United States.)

In other words, nearly two thirds of those who tested positive for kratom also had fentanyl or an analog in their bodies. Heroin was found in 32.9 percent of the kratom-positive cases, benzodiazepines, prescription opioids, and cocaine were also found.

“The fact that fentanyl was showing up in so many of these cases suggests to me that people are using street opioids in conjunction with kratom,” Prozialeck says. That makes it much harder to reasonably assess the impact of kratom itself, as the CDC says it wants to do. The presence of other—potentially deadly—substances complicates the picture. “I think the CDC might be overstating the cause-effect relationship between kratom and these reported deaths,” Prozialeck says.

When asked for comment, a rep from the CDC said, "Kratom was detected on post-mortem toxicology for 152 decedents. In about 60 percent of those deaths, the medical examiner or coroner determined that kratom contributed to the death. That said, 84 of the 91 decedents had other substances detected on post-mortem toxicology, and most of those 84 had fentanyl, heroin, benzodiazepines, or cocaine."

For this report, the CDC's data was collected by the State Unintentional Drug Overdose Reporting System (SUDORS), which collates stats on unintentional and undetermined-intent opioid overdose deaths at the local level across the US, as documented by a medical examiner or a coroner.

SUDORS records data on overdoses in which at least one opioid contributed to the death, as well as fatal overdoses with no contributing opioid if substances that have opioid-like properties contributed to death. (The FDA maintains that kratom has "opioid-like" properties.) For all included deaths, the system records all substances on postmortem toxicology testing, including those that did and did not contribute to the death.

There are a lot of unknowns about kratom right now, Prozialeck says. With these cases, “we don’t know what people were actually taking.” They could have knowingly taken kratom alongside other substances, or, in part because of its current, unregulated status, unwittingly used kratom laced with something lethal. We also don’t know how kratom may affect people with seizure disorders, mental health issues, or heart issues, for example. Answering those questions will require research that has yet to be done.

Update: This story has been updated to reflect a response from the CDC.