Psychedelic substances, long used in traditional healing and religious ceremonies, are now being studied for therapeutic use. This review examines evidence from quantitative, qualitative, and neurobiological studies on psychedelic-assisted therapy, focusing on the hypothesis that psychedelics facilitate the emergence of emotionally charged unconscious material, originally proposed by Stanislav Grof. Process studies and qualitative reports indicate that the psychedelic experience is linked to increased subjective access to and awareness of emotions, memories, and perceptions normally outside conscious awareness. Brain-imaging studies show several neurobiological effects that may relate to these experiences. Available evidence suggests this process could be an important therapeutic mechanism, warranting further investigation.
Psychedelic-assisted psychotherapy (PAP) shows large to very large effects on symptoms of depression and anxiety at post-treatment and follow-up, with Hedge's g values ranging from 0.98 to 1.83. A meta-analysis of 23 clinical trials found some evidence that two dosing sessions produce larger between-group effects than one session, though this finding was not robust across all analyses and needs replication. Only three studies used evidence-based psychotherapy protocols, limiting analysis of that moderator. The field would benefit from more thorough reporting of psychotherapy protocols to enable comparison between studies.