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 Gottleib 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.

Kratom has been linked to at least four deaths in the Philadelphia region. In Pennsylvania, mitragynine was present in 27 deaths in 2017, according to a statewide database that includes information from 37 of the state's 67 county coroners.

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 several U.S. states and municipalities.