The U.S. Food and Drug Administration issued an advisory Tuesday after finding "disturbingly high levels" of heavy metals in the herbal product kratom.

Field scientists tested 26 separate kratom products during a multistate outbreak of salmonella infections and during other investigations. They found lead and nickel at "levels not safe for human consumption," the FDA stated.

"While the levels of the specific products we've tested so far are not likely to result in immediate acute heavy metal poisoning from a single use, some of these products included levels that, with chronic use, could cause some people to suffer from heavy metal poisoning," Commissioner Scott Gottlieb said.

Kratom, derived from the leaves of a Southeast Asian tree that is part of the coffee family, has gained popularity in recent years. The FDA has said the active ingredient in kratom, mitragynine, is an addictive substance that acts on the brain's opioid receptors.

In April, salmonella infections linked to kratom sickened at least 132 people in 38 states, the Centers for Disease Control and Prevention reported at the time.

In September, two manufacturers were issued warnings for allegedly making unproven medical claims that the herbal product would "relieve opium withdrawals" and treat medical conditions including diarrhea, depression, diabetes, obesity, high blood pressure, stomach parasites, diverticulitis, anxiety, and alcoholism.

To date, there have been no "adequate and well-controlled scientific studies" involving the use of kratom as a treatment for medical conditions or diseases in humans, the FDA warning stated.

Kratom has been linked to at least four deaths in the Philadelphia region. In Pennsylvania, mitragynine was present in 27 deaths last year, according to a statewide database that includes information from 37 of the state's 67 county coroners.

Kratom is sold online, in gas stations and smoke shops, and is typically brewed as a tea, chewed, smoked, or ingested in capsules. It is banned in several countries, including Australia, Denmark, Germany, Malaysia, and Thailand, as well as several U.S. states and municipalities.