Imperial College London Psilocybin Depression-Treatment Feasibility Study

The Imperial College London Psilocybin Depression-Treatment Feasibility Study was an early clinical test of psilocybin for treatment-resistant depression. Directed by Robin Carhart-Harris in David Nutt’s lab, it produced a striking but preliminary signal: symptoms improved rapidly for many participants, while the study’s small scale and lack of a control group left important questions about the treatment’s effects unanswered.

Study design and participants

The initial study enrolled twelve people—six men and six women—with treatment-resistant depression, defined as having tried at least two treatments without success. The initial results appeared in Lancet Psychiatry in 2016. The study had no control group, and participants knew they were receiving psilocybin, so it was an open trial rather than a randomized comparison with another treatment.

Early results and follow-up

After one week, all twelve volunteers showed improvement in their symptoms. Two-thirds were depression-free at that point, in some cases for the first time in years. At three months, seven of the twelve continued to show substantial benefit.

The study was then expanded to a total of twenty participants. At six months, six remained in remission; the others had relapsed to varying degrees. That pattern suggested the treatment might need to be repeated. The results therefore combined a rapid initial change with a less uniform longer-term course, rather than establishing that one session produced lasting remission for everyone.

Limits and influence

The researchers described the work as a modest feasibility study, not a definitive test of efficacy. Its small sample, open design, and lack of randomization limit what can be concluded about how much of the improvement was caused by psilocybin. At the same time, the treatment was well tolerated in this group, no adverse events were reported, and most participants experienced marked and rapid benefits.

The European Medicines Agency considered this study alongside results from research at Johns Hopkins University and NYU. Although regulators had initially been approached about psilocybin for anxiety and depression in people facing life-changing diagnoses, they noted that existential distress was not an official DSM diagnosis and would not be covered by national health services. The early depression findings nevertheless suggested a broader clinical possibility, and the agency recommended a much larger trial for treatment-resistant depression. The study’s significance thus lay both in its encouraging outcomes and in how a small, preliminary experiment helped redirect the question toward larger-scale testing.