Psilocybin, the psychedelic compound in ‘magic’ mushrooms, may be an effective way to treat depression in patients that have seen no benefit from other, standard forms of treatment, early results suggest.

In a pilot study involving just 12 people with treatment-resistant depression, two doses of the mushroom compound cleared symptoms in eight participants—67 percent—after one week. After three months and no other doses, seven participants still reported reduced depressive symptoms, including five—42 percent—who reported complete remission of their depression.

But the finding, published Tuesday in the Lancet Psychiatry, is just a first step to assess the safety of using the hallucinogenic drug. With such a small study, no controls, and non-randomized participants, it is not possible to determine if the promising efficacy results will stand.

“We are simply saying that this is doable,” Robin Carhart-Harris, a neuropsychopharmacologist at Imperial College London and first author of the study, told Nature. “We can give psilocybin to depressed patients, they can tolerate it, and it is safe.”

Carhart-Harris and colleagues got the idea to try psilocybin after earlier brain imaging studies found that the compound activated brain regions associated with antidepressant effects. Plus, large population studies have found that people who used psychedelics in their lifetime have lower rates of psychological distress and suicidal episodes compared to those that didn’t.

Moreover, psilocybin is readily converted to psilocin in the body, which can activate serotonin receptors. These receptors, which influence a wide range of biological processes, such as learning, memory, and mood, are also targeted by more standard depression treatments—namely, selective serotonin reuptake inhibitors (SSRIs) such as Celexa and Prozac.

After an arduous approval process for the initial trial, the research team enrolled the 12 participants, who had moderate to severe depression and had seen no benefits from at least two other treatments. With psychiatrists standing by, researchers started participants off with just a small dose of 10 milligrams of psilocybin to test tolerance. A week later, they gave participants a 25mg dose, which was considered the test amount.

Psilocybin has very low toxicity and little risk of addiction. During the study, the authors reported no adverse effects during the trial aside from mild, transient anxiety, nausea, and headaches.

While the efficacy was “pretty remarkable,” according to Carhart-Harris, far more work will need to be done to prove that psilocybin can treat depression reliably. For one thing, five of the 12 participants had been exposed to the drug before and may be more tolerant to the drug and positive about the experience going into the study.

Overall, the study’s finding “motivates further trials, with more rigorous designs, to better examine the therapeutic potential of this approach,” the authors conclude.

Lancet Psychiatry, 2016. DOI: 10.1016/S2215-0366(16)30065-7 (About DOIs).