Ketamine leads something of a double life, straddling the line between medical science and party drug. Since its invention in the early 1960s, ketamine has enjoyed a quiet existence as a veterinary and pediatric anesthetic given in high doses. But in a second, wilder life, ketamine’s effects at lower doses—a profound sense of dissociation from self and body—became an illicit favorite among psychedelic enthusiasts. Pioneering neuroscientist John Lilly, who famously attempted to facilitate communication between humans and dolphins, used the drug in the late 1970s during experiments in sensory deprivation tanks. By the 1990s, the drug had made its way to the dance floor as “special K.”

More recently, ketamine has taken on a third, wholly unexpected role. Since the early 2000s, the drug has been studied as a uniquely powerful medication for treating severe depression and obsessive-compulsive disorder (OCD). When given as an intravenous infusion, ketamine can lift symptoms of depression and OCD from patients who fail to respond to common antidepressants like Prozac and even resist treatments like electroconvulsive therapy (ECT).

Exactly how ketamine produces antidepressant effects remains unclear, however. Antidepressants like Prozac are Serotonin Reuptake Inhibitors (SSRIs) that increase levels of the neurotransmitter serotonin in the brain, which is believed to boost mood. Ketamine’s main mechanism of action to produce dissociative anesthetic effects, on the other hand, depends on another neurotransmitter, glutamate.

“The prevailing hypothesis for ketamine’s antidepressant effect is that it blocks a receptor (or docking port) for glutamate,” says Carolyn Rodriguez, a professor of psychiatry at Stanford who has conducted some of the pioneering research into ketamine as an OCD treatment.

However, new research suggests that ketamine’s influence on glutamate receptors, and specifically the NMDA receptor, may not be the sole cause of its antidepressant effects. According to a recent study in the American Journal of Psychiatry by Rodriguez and her Stanford colleagues, ketamine might also activate a third system in the brain: opioid receptors.

Ketamine is known to bind weakly to the mu opioid receptor, acting as an agonist to produce a physiological response at the same site in the brain where narcotics like morphine exert their influence. It’s also known that opioids can have antidepressant effects, says Alan Schatzberg, a professor of psychiatry at Stanford and co-author of the new study.

It never made sense to Schatzberg that ketamine’s antidepressant effects were a result of blocking the glutamate receptors, as attempts to use other glutamate-blocking drugs as antidepressants have largely failed. The Stanford psychiatrist, who has spent his career studying depression, wondered if researchers were unknowingly activating opioid receptors with ketamine.

“You could test this by using an antagonist of the opioid system to see if you blocked the effect in people who are ketamine responders,” he says. “And that’s what we did.”

The researchers enlisted 12 subjects with treatment-resistant depression and gave them either an infusion of ketamine preceded by a placebo, or ketamine preceded by a dose of naltrexone, an opioid receptor blocker. Of those, seven subjects responded to the ketamine with placebo, “and it was very dramatic,” Schatzberg says, with depression lifting by the next day. “But in the other condition, they showed no effect,” suggesting it was the opioid receptor activity, not blocking glutamate receptors, that was responsible.

While opioid blockers prevented ketamine from activating the associated receptors, it did not block the drugs dissociative effects, suggesting dissociation alone won’t affect depression. “It’s not that, ‘hey, we’ll get you a little weird and you’ll get the effect,’” Schatzberg says.

The appeal of ketamine’s use as an antidepressant is clear enough. While more typical antidepressants may require six to eight weeks to produce benefits, ketamine works within hours.

“Our patients are asked to hang in there until the medication and talk therapy takes effect,” says Carlos Zarate, chief of the experimental therapeutics and pathophysiology branch of the National Institute of Mental Health (NIMH) who was not associated with the new study. While waiting for traditional treatments to kick in, patients “may lose their friends or even attempt suicide.”

A treatment that works within 24 hours? “That’s huge.”

But the study linking ketamine to opioid activity means an extra dose of caution is required. While ketamine acts quickly, the anti-depressive effects of the drug only last for a few days to a week, meaning repeat doses would be needed in practice. Researchers and clinicians should consider the risk of addiction in long-term use, Schatzberg says. “You’re going to eventually get into some form of tolerance I think, and that’s not good.”

However, the new finding is based on just seven subjects, and it still needs to be replicated by other scientists, says Yale professor of psychiatry Greg Sanacora, who was not involved in the new study. And even if the trial is replicated, it would not prove ketamine’s opioid activity is responsible for its antidepressant effects.

“It doesn’t show that at all,” says Sanacora, who studies glutamate, mood disorders and ketamine. “It shows that the opioid system needs to be functioning in order to get this response.”

Sanacora compares the new study to using antibiotics to treat an ear infection. If you administered an additional drug that blocks absorption of antibiotics in the stomach, you would block treatment of the ear infection, but you wouldn’t conclude that antibiotics fight ear infections through stomach absorption—you just need a normally functioning stomach to allow the antibiotic to do its job. Similarly, opioid receptors might need to be functioning normally for ketamine to produce antidepressant effects, even if opioid activity is not directly responsible for those effects.

Complicating matters further, placebos often cause patients to experience less pain, but opioid blockers like naltrexone have been shown to prevent this response, according to Sanacora. It could be, he suggests, that all the apparatus of the clinic—the nursing staff, the equipment—exerted a placebo effect that is mediated by the brain’s opioid system, and the patients who received naltrexone simply did not respond to that placebo effect.

“That’s a very important and powerful tool that is in all of medicine, not just in psychiatry,” Sanacora says. “And we know that the opiate system is involved, to some extent, in that type of response.”

It’s also possible, the researchers note in the paper, that ketamine’s action at the glutamate receptor is still important. “Ketamine acts in three distinct phases—rapid effects, sustained effects and return to baseline,” Rodriguez says. Opioid signaling may turn out to mediate ketamine’s rapid effects, while “the glutamate system may be responsible for the sustaining effects after ketamine is metabolized.”

One interpretation is that ketamine blocks glutamate receptors on neurons that are inhibitory, meaning they signal other neurons to fire fewer signals. By blocking these neurons from firing, ketamine may enhance glutamate activity in the rest of the brain, producing anti-depressive effects that persist after the opioid activity dies down.

“The reality is it’s in a gray zone,” Sanacora says. “This is just one small piece of a very large puzzle or concern that we really need to look at the data in total.”

That data is forthcoming. Results from a Janssen Pharmaceuticals clinical trial using esketamine, an isomer of ketamine, and involving hundreds of subjects will soon become public, according to Sanacora, who has consulted for the company. And at NIMH, Zarate and colleagues are studying hydroxynorketamine, a metabolite of ketamine that may provide the same benefits but without the dissociative side effects.

The ultimate goal of all this research is to find a ketamine-like drug with fewer liabilities, and that aim is bringing researchers back to the fundamentals of science.

“For me, one of the exciting parts of this study is that it suggests that ketamine’s mechanism is complicated, it acts on different receptors beyond glutamate and is the start of this exciting dialogue,” Rodriguez says. “Sometimes great science raises more questions than answers.”