An apparent spike in the use of an unapproved antidepressant called tianeptine is poisoning people who are looking to benefit from the drug’s mild opioid-like effects, according to a Centers for Disease Control and Prevention (CDC) report released last week.

From 2000 to 2013, the U.S. saw 11 tianeptine-related poison control center calls; from 2014 to 2017, there were more than 200. That’s all according to the CDC analysis of National Poison Data System information, which sheds new light on a growing trend.

Though tianeptine isn’t FDA-approved, it’s not illegal and can be purchased online as a dietary supplement. It’s often marketed elsewhere under brand names Coaxil and Stablon, according to Vice News.

The drug was first discovered by the French Society of Medical Research back in the 1960s, and it’s been shown to help fight depression and anxiety, according to CNN.

When it was patented, scientists weren’t entirely clear on how it worked. But in 2014, researchers found that the drug lights up certain opioid receptors. While that appears to help with some depression symptoms, it also means that people taking tianeptine can have opioid-like withdrawal when they stop taking the drug.

"Tianeptine has an abuse potential in former opiate drug users," the CDC researchers wrote. "This study further highlights that the withdrawal effects of tianeptine mimic those of opioid withdrawal."

But, unlike with traditional opioids of abuse, tianeptine doesn’t show up on drug screens, which can make it a tempting choice for justice-involved individuals looking for a high that won’t land them in legal hot water.

"I think people have this misguided belief that if you can get it on the internet and it's not overtly illegal and you're not going through the dark web to acquire these substances, so it must be OK," Raphael Leo, an associate professor in the Department of Psychiatry at the University of Buffalo, told CNN.

But, earlier this year, the Journal of Analytical Toxicology reported on two tianeptine-related deaths in Texas, and more have been reported in other countries.

The potential dangers of the unapproved drug have sparked some calls to ban it. In April, Michigan greenlit a law banning the substance on the heels of a number of overdoses, according to the Associated Press.