The herbal drug kratom led to 91 overdoses between July 2016 and December 2017, according to a new report by the Centers for Disease Control and Prevention.

In about 80 percent of the deaths linked to kratom during that period, the person had a history of using other drugs, USA Today reports. In most of the overdoses, fentanyl was also listed as a cause of death.

Last month, researchers reported that poison control centers are receiving a growing number of calls related to kratom. There were more than 1,800 calls related to kratom exposures from January 2011 through December 2017. Sixty-five percent of the calls were made in 2016 and 2017.

The supplement is not regulated by the Food and Drug Administration because it is not considered a drug. Kratom is legal in most states. It is taken to relieve pain or ease the effects of opioid withdrawal. Some people take it to boost their mood or help with anxiety or depression.