The U.S. Food and Drug Administration issued a new warning Tuesday to two companies that are making unproven medical claims about the herbal product kratom.

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.

The FDA sent warning letters to Chillin Mix Kratom and Mitra Distributing over claims that the herbal product would “relieve opium withdrawals” and treats 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 warning stated.

“Fraudulent health claims can pose serious health risks,” FDA Commissioner Scott Gottlieb said in the announcement. Consumers who rely on such products may not seek appropriate therapies and they may be at risk of overdose and death, he stated.

The product is sold online, in gas stations and in 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 in several U.S. states and municipalities.