The Food and Drug Administration released a report on February 6 warning people against using the herbal pain reliever kratom, saying it could pose the same addictive qualities as morphine.
Gottlieb said the FDA has been especially concerned about kratom's use to treat opioid withdrawal symptoms, and warned against it.
The Food and Drug Administration declared "Kratom" to be an opioid this week, due to its potential for abuse, addiction and serious health consequences, including death.
The substance, which is imported from Southeast Asia and marketed as a supplement, has become increasingly popular among consumers looking for relief from pain, anxiety and depression as well as opioid-withdrawal symptoms.
"We recognize that many people have unmet needs when it comes to treating pain or addiction disorders", he said.
In the advisory released in November, Dr. Gottlieb referenced 36 reported deaths linked to use of kratom-containing products, and cited instances of kratom laced with other opioids like hydrocodone.
In many of those cases, kratom was used along with other drugs, making it hard for authorities to determine the actual cause of death. The scientific data and adverse event reports have "clearly revealed" that compounds in kratom render it more unsafe than "just a plant", he said.
However, researchers who study kratom were quick to point out that the FDA is making too broad a statement. But officials backtracked after a public outcry and pressure from some members of Congress.
The FDA statements both emphasized that the FDA encourages controlled, scientific research to fully investigate the properties and possible therapeutic uses of kratom. The plant is not now regulated, and the FDA is concerned about the plant's potential for abuse, addiction, and death. The model, he said, can predict how a substance will behave in the body and affect various receptors in the brain based on its chemical structure and its similarity to drugs for which there is data. The model also showed that 22 of the compounds bind strongly to opioid receptors in the brain and to receptors "that may contribute to stress responses that impact neurological and cardiovascular function".
"Patients addicted to opioids are using kratom without dependable instructions for use and more importantly, without consultation with a licensed health care provider about the product's dangers, potential side effects or interactions with other drugs", Gottlieb said in a previous statement in November.
In all but one case, the people who died were found to have been taking multiple drugs, including other opioids in many cases.
Although a number of deaths have been linked to use of kratom, it's unclear if the deaths are a direct result of using the drug, Marc Swogger, an associate professor of psychiatry at the University of Rochester Medical Center in NY, told Live Science in a 2016 interview.