A new small study finds the hallucinogenic drug psilocybin was more effective than standard antidepressants in treating major depressive disorder, NPR reports.

The study of 27 people found psilocybin had an effect that was more than four times greater than standard antidepressant treatment. The findings are published in JAMA Psychiatry.

Dr. Charles F. Reynolds III of the University of Pittsburgh School of Medicine, who wrote an editorial accompanying the study, said the study offers “a good deal of promise as a feasible approach to treating particularly chronic forms of depression.” He noted that the results might be skewed because patients were told they were going to get the drug.

“Some of the rapid improvement that we saw could have been related to expectancy effects on the part of the participants,” he said.

Under federal law, it is illegal to grow or possess psilocybin without a special license from the Drug Enforcement Administration.