What does the long-term follow-up evidence actually show?
The most direct long-term evidence comes from a systematic review that included two long-term follow-up reports from the same trial [1]. In one small study of 31 people with alcohol use disorder, 32% (10 out of 31) achieved complete abstinence from alcohol, and this was measured over an average follow-up period of 6 years [1]. That means nearly a third of participants in that study stayed completely sober for years after psilocybin-assisted therapy. In another pilot study of 15 smokers, 67% (10 out of 15) were still not smoking at 52 weeks (one year) after treatment [1]. These are striking numbers, but they come from very small groups — 31 and 15 people respectively — so we can't assume they apply to everyone.
A broader meta-analysis of psilocybin therapy for depression found that the overall treatment effect was actually larger at long-term follow-up (Cohen's d = 2.10) than in the short term (Cohen's d = 1.69) [2]. Cohen's d is a measure of effect size; a d of 2.10 is considered very large, meaning the improvement was substantial. However, this analysis included studies with follow-up periods that varied, and the authors caution that the results are based on small sample sizes and inconsistent reporting [2]. So while the long-term effect looks strong, the evidence base is still thin.
What's missing from the long-term picture?
Despite the promising signals, the long-term evidence has major gaps. The most comprehensive systematic review on psilocybin for addiction found only four studies total, and only one of them was a randomized controlled trial (RCT) [1]. The rest were small pilot studies or single-arm trials without a placebo group, which makes it hard to rule out that improvements were due to other factors like natural recovery or the psychotherapy component alone. The authors explicitly call for larger RCTs with longer follow-up [1].
The FDA's recent decision adds another layer of caution. In August 2024, the FDA declined to approve MDMA for PTSD and asked the sponsor to conduct another phase 3 trial [4]. This means that even after years of research, the regulatory body did not consider the existing evidence — including long-term data — strong enough to approve the treatment. This underscores that while the early results are exciting, the long-term evidence is not yet robust enough for widespread clinical use.
There is also a lack of standardized methods for tracking long-term harms. A 2024 framework paper identified 54 potential adverse events that could occur after the acute psychedelic experience, including lasting changes in worldview or spirituality, and found that existing measurement tools miss many of these [5]. Without good long-term safety data, we cannot fully assess the risk-benefit balance over years.
About These Sources
This answer is built on 5 peer-reviewed studies — published from 2023 to 2025, 4 from 2024 or later, 4 in Q1 journals, collectively cited 152 times — selected as the most relevant from 5 studies that passed quality screening, drawn from 68 papers retrieved from a database of over 500 million.
Sources used in this answer
Therapeutic effect of psilocybin in addiction: A systematic review
In a systematic review of psilocybin for substance use disorders, only four studies were found (one RCT, three small trials); long-term follow-up showed 32% of alcohol patients abstinent at 6 years and 67% of smokers abstinent at 52 weeks, but the authors call for larger RCTs.
A dose of therapy with psilocybin - A meta-analysis of the relationship between the amount of therapy hours and treatment outcomes in psychedelic-assisted therapy
A meta-analysis of psilocybin for depression found a very large treatment effect at long-term follow-up (Cohen's d = 2.10), but no significant relationship between therapy hours and outcome, and the authors note small samples and poor reporting.
The value of biomarkers in the therapy of CRSwNP with biologicals—a long-term follow-up of dupilumab therapy
This paper is about dupilumab for chronic sinusitis, not psychedelic therapy, and is not relevant to the question.
Psychedelic-assisted therapy: An overview for the internist
An overview for internists notes that the FDA declined to approve MDMA for PTSD in August 2024, asking for another phase 3 trial, indicating that long-term evidence was not yet sufficient for approval.
A framework for assessment of adverse events occurring in psychedelic-assisted therapies
A framework paper identified 54 potential adverse events in psychedelic therapy that are not well captured by current measures, highlighting gaps in long-term safety monitoring.