THEPSYCHEDELICLIBRARY

Chapter 3

What the evidence says

An honest overview of where the research is strong, where it's promising and where it's thin.

ReviewedLast updated September 26, 2026

In short: the strongest evidence is for ketamine and esketamine in treatment-resistant depression. Psilocybin and MDMA have promising results from randomized trials, with real limitations. For most other substances, the human evidence is early: small studies, surveys and case reports.

How to read a study

Not all studies are equal. Here’s a rough ladder, from weakest to strongest:

  1. Case reports and case series. Descriptions of one or a few people. Useful for spotting rare side effects, but they can’t show a treatment works.
  2. Surveys and observational studies. Researchers ask or observe people who used a substance on their own. The people who respond are self-selected, and there’s no comparison group, so these studies can show links but not cause and effect.
  3. Open-label trials. Everyone gets the treatment and knows it. Good for testing safety and feasibility.
  4. Randomized controlled trials (RCTs). People are randomly assigned to the treatment or a comparison (placebo, a low dose, or another treatment). This is the best way to test whether a treatment itself causes an effect.
  5. Systematic reviews and meta-analyses. Studies that pool the results of many trials. Only as good as the trials they include.

The blinding problem

In most drug trials, neither participants nor researchers know who got the real drug. That’s called blinding, and it stops expectations from shaping the results.

With psychedelics, blinding is very hard. Most people can tell within an hour whether they received an active dose. People who hope for a breakthrough and realize they got the placebo may feel disappointed and report feeling worse, which makes the real drug look better by comparison. Researchers are testing fixes, such as comparing different doses or using active placebos, but it’s a limitation to keep in mind in almost every study in this field.

Where the evidence stands, by molecule

Ketamine and esketamine: the most established. Many randomized trials show fast relief in treatment-resistant depression. Esketamine is approved in Canada, the US and elsewhere. In a 2023 trial of 403 people, IV ketamine was at least as effective as electroconvulsive therapy.1 The main open question is how to keep the benefits going.

Psilocybin: promising, with mixed large-trial results. In a phase 2 trial of 233 people with treatment-resistant depression, a single 25 mg dose improved depression scores more than a 1 mg dose at three weeks, but the difference wasn’t statistically clear by 12 weeks.2 In a trial of 59 people comparing psilocybin with the antidepressant escitalopram, the main measure didn’t differ significantly between the groups.3 Smaller trials in people with cancer-related distress4 and alcohol use disorder5 have reported encouraging results.

MDMA: strong trial results, and a regulatory setback. Two phase 3 trials of MDMA-assisted therapy for PTSD, with 90 and 104 participants, reported larger improvements than therapy with a placebo.6 In 2024, the US Food and Drug Administration declined to approve it and asked for another trial, citing concerns including blinding and how the studies were conducted.

LSD: growing. Newer trials are studying LSD for anxiety and depression. Evidence is earlier-stage than for psilocybin.

DMT, 5-MeO-DMT, ayahuasca, ibogaine, mescaline, 2C-B and salvia: early. Most human evidence comes from small studies, surveys of people using these substances outside of clinics, and case reports. Some, especially ibogaine, have serious safety concerns (see Safety).

How we grade evidence on this site

Every study summary in our study library carries a grade describing that single study:

  • High: large, well-run randomized trials, or pooled analyses of them with consistent results.
  • Moderate: randomized trials with important limitations, such as small size, short follow-up or blinding problems.
  • Low: open-label trials, secondary analyses, observational studies and surveys.
  • Very low: case reports, case series, very small pilots, or too little information to judge.

A grade isn’t a verdict on the substance. It tells you how much weight one study can carry. For condition-by-condition summaries, see Evidence by condition.

Sources

  1. Anand A, et al. Ketamine versus ECT for nonpsychotic treatment-resistant major depression. New England Journal of Medicine, 2023. PubMed
  2. Goodwin GM, et al. Single-dose psilocybin for a treatment-resistant episode of major depression. New England Journal of Medicine, 2022. PubMed
  3. Carhart-Harris R, et al. Trial of psilocybin versus escitalopram for depression. New England Journal of Medicine, 2021. PubMed
  4. Griffiths RR, et al. Psilocybin produces substantial and sustained decreases in depression and anxiety in patients with life-threatening cancer. Journal of Psychopharmacology, 2016. PubMed
  5. Bogenschutz MP, et al. Percentage of heavy drinking days following psilocybin-assisted psychotherapy vs placebo in the treatment of adult patients with alcohol use disorder. JAMA Psychiatry, 2022. PubMed
  6. Mitchell JM, et al. MDMA-assisted therapy for severe PTSD. Nature Medicine, 2021; and Mitchell JM, et al. MDMA-assisted therapy for moderate to severe PTSD. Nature Medicine, 2023. PubMed