Evaluation of Psilocybin-assisted Psychotherapy (PaP) for the Treatment of Post-traumatic Stress Disorder (PTSD) in Military Veterans


Post-Traumatic Stress Disorder (PTSD) is a mental health condition that occurs as a result of a traumatic experience. Symptoms include feeling anxious, flashbacks, nightmares and difficulty sleeping. Several studies indicate that psilocybin-assisted psychotherapy (PaP) may be an effective treatment for a number of mental health conditions. This has led to PaP being designated as a "breakthrough treatment" by the FDA in the US. Despite indications that PaP may hold benefits in treating individuals with posttraumatic stress disorder (PTSD), this remains to be investigated. As such, the present study aims to examine the acceptability, feasibility, safety, and efficacy of PaP (psilocybin administered with psychotherapy) in treating PTSD in military veterans.

Full Title of Study: “Evaluation of the Acceptability, Safety, Feasibility, and Efficacy of Psilocybin-assisted Psychotherapy (PaP) for the Treatment of Post-traumatic Stress Disorder (PTSD) in Military Veterans”

Study Type

  • Study Type: Interventional
  • Study Design
    • Allocation: N/A
    • Intervention Model: Single Group Assignment
    • Primary Purpose: Treatment
    • Masking: None (Open Label)
  • Study Primary Completion Date: July 2025

Detailed Description

Recent studies have shown that Psilocybin-Assisted Psychotherapy (PaP) for individuals with treatment-resistant depression can result in outcomes that exceed routine psychotherapy. Psilocybin may have a catalytic effect on the psychotherapeutic process, enhancing introspection and interoception. PaP may similarly benefit the treatment of posttraumatic stress disorder (PTSD). Research indicates high treatment drop-out rates (approximately 30%) among PTSD patients, and moderate remission rates (approximately 44%) 40 months after completing treatment. Furthermore, some veterans with PTSD have poorer treatment responses than members of the general public. This suggests that alternative treatment approaches may be required to support veterans who do not benefit from standard evidence-based approaches. This study aims to explore the acceptability, feasibility, safety and efficacy of PaP for veterans with PTSD. A total of eight military veterans will be recruited. The study involves two non-directive preparatory sessions, two dosing sessions of psilocybin, followed by 12 sessions of Cognitive Processing Therapy. It is hypothesised that PaP will result in a significant reduction in PTSD symptoms, as indicated by PTSD Checklist for Diagnostic and Statistical Manual of Mental Disorders (DSM-5; PCL-5) scores from baseline to one-month follow-up.


  • Drug: Psilocybin
    • Product name: Psilocybin Pharmaceutical form: capsule, hard Dose number and units: 25 mg per day (8-hour dosing session) x 2 Route of administration: oral

Arms, Groups and Cohorts

  • Experimental: Psilocybin-assisted Psychotherapy
    • All participants will receive 25mg psilocybin (capsule, hard, oral administration) in two 8-hour psilocybin dosing sessions, followed by Cognitive Processing Therapy.

Clinical Trial Outcome Measures

Primary Measures

  • Symptoms of PTSD measured using the Posttraumatic Stress Disorder Checklist For DSM-5 (PCL- PTSD symptoms
    • Time Frame: Change from baseline PCL-5 score at one month follow up
    • Symptoms of PTSD measured using the Posttraumatic Stress Disorder Checklist For DSM-5 (PCL- 5). Scores range from 0-80, with a higher score indicated a worse outcome.

Secondary Measures

  • Core features of PTSD and complex PTSD measured using the International Trauma Questionnaire (ITQ)
    • Time Frame: Change from baseline ITQ score at one month follow up
    • Scores range from 0 to 48 with a higher score indicating a worse outcome.
  • Difficulties with anger measured using the Dimensions of Anger Reactions (DAR-5)
    • Time Frame: Change from baseline DAR-5 score at one month follow up
    • Scores range from 5-25, with a higher score indicating a worse outcome.
  • Depression symptoms measured using the Patient Health Questionnaire (PHQ-9)
    • Time Frame: Change from baseline PHQ-9 score at one month follow up
    • Scores range from 0-27, with a higher score indicating a worse outcome.
  • General anxiety symptoms measured using the Generalised Anxiety Disorder (GAD-7)
    • Time Frame: Change from baseline GAD-7 score at one month follow up
    • Scores range from 0-14, with a higher score indicating a worse outcome.
  • Mental wellbeing measured using the Short Warwick-Edinburgh Mental Wellbeing Scale (SWEMWBS)
    • Time Frame: Change from baseline SWEMWBS score at one month follow up
    • Scores range from 7-35, with a lower score indicating a worse outcome.
  • Perceived social support measured using the Oslo Social Support Scale
    • Time Frame: Change from baseline OSS score at one month follow up
    • Scores range from 3-14, with a higher score indicating a worse outcome.
  • Challenging aspects of experiences with psilocybin measured using the Challenging Experience Questionnaire
    • Time Frame: Administered at the end of dosing session one, week 4
    • Scores range from 5-25, with a higher score indicating a worse outcome.
  • Challenging aspects of experiences with psilocybin measured using the Challenging Experience Questionnaire
    • Time Frame: Administered at the end of dosing session two, week 5
    • Measures a phenomenological profile of experiences with scores not indicative of more or less strongly challenging experiences.

Participating in This Clinical Trial

Inclusion Criteria

1. Aged 18-65 years 2. Fluent in English (reading and speaking) 3. Has internet access via computer or tablet 4. Is able to commit to the study visits and treatment length 5. Can provide a contact (relative, close friend, other support person) who is able to accompany the participant to dosing visits 6. Agrees to inform researchers within 48 hours of any medical treatments or procedures 7. Can swallow pills 8. Agrees to lifestyle restrictions: not to consume alcohol within 24 hours prior to dosing, and to not consume more caffeine than usual 9. Agrees to not participate in any other clinical trials for the duration of the study 10. PCL-5 score ≥33 11. At least one unsuccessful evidence-based psychotherapy/pharmacotherapy for PTSD Exclusion Criteria:

General exclusion criteria:

1. History of poor cooperation or unreliability 2. Engaged in compensation litigation whereby financial game would be achieved from prolonged symptoms of PTSD or any other psychiatric disorders 3. Any other current problems that may interfere with participation (e.g., availability, private space for sessions at home) 4. Has hearing impairment that could interfere with ability to participate in the study 5. Is unable to provide written informed consent 6. Has known hypersensitivity or previous allergic reaction to any constituent of psilocybin 7. Pregnant or breastfeeding 8. BMI <18 or >35 or non-consent for metric to be measured during assessment visit 9. Has been diagnosed with, or has first degree relative with schizophrenia, psychotic disorder (unless substance induced or due to medical condition), or bipolar I disorder 10. Current alcohol or substance use disorder (other than caffeine or nicotine) requiring detox, or currently in withdrawal from such disorder. Exception for milder disorder if realistic plan (agreed by researcher, therapy team, and medical monitor) for successfully mitigating alcohol/substance use to prevent use from impacting participation, safety, and/or efficacy of the treatment. Mental health exclusion: 11. Schizophrenia spectrum or other psychotic disorders or first degree relative with such disorders (incl. major depressive disorder with psychotic features, or Bipolar I or II disorders) 12. May present serious risk to others (established via clinical interview and contact with treating psychiatrist) 13. Is likely to be re-exposed to index trauma or other significant trauma, lack social support, or lack of stable living situation Physical health exclusion: 14. History of myocardial infarction, angina, cerebrovascular accident, aneurysm, or pulmonary vascular disease 15. Has had Transient Ischemic Attack (TIA) within past 6 months 16. Has uncontrolled hypertension (140/90 mmHG or higher assessed on three separate occasions). Adequately controlled hypertension does not exclude participant 17. Has Wolff-Parkinson-White syndrome or other accessory pathway conditions that have not been successfully eliminated by ablation 18. History of arrhythmia, other than premature atrial contractions (PACs) or occasional premature ventricular contractions (PVCs) in the absence of ischemic heart disease, within past 12 month 19. History of risk factors for Torsade de pointes (e.g., heart failure, hypokalemia, Long QT Syndrome family history) 20. Requires use of concomitant medications that may prolong the QT/QTc interval during psilocybin dosing sessions 21. Marked Baseline prolongation of QT/corrected QT interval (QTc; e.g., repeated demonstration of a QTc interval >450ms and >460ms in females corrected by Bazett's formula). For transgender or non-binary subjects, QTc interval will be evaluated based on sex assigned at birth, unless the subject has been on hormonal treatment for five or more years. 22. History of medical condition that could make receiving a sympathomimetic drug harmful because of increased blood pressure and heart rate 23. Haptic enzymes alkaline phosphatase (ALP), alanine transaminase (ALT), Aspartate aminotransferase (AST) or Gamma-glutamyl Transferase (GGT) > three times upper limit of normal (ULN), or total bilirubin levels >2x ULN 24. Previous indication of liver or kidney damage 25. Current Hepatitis C virus (HCV) infection – Asymptomatic HCV permitted if previously undergone evaluation and treatment as needed 26. Current uncontrolled Type 2 diabetes mellitus 27. Current uncontrolled hypothyroidism 28. Current or historic glaucoma unless participation approved by an ophthalmologist 29. History of traumatic brain injury (TBI)/cognitive impairment limiting ability to engage in treatment (e.g., memory or concentration problems, impulsivity related to brain injury) 30. Current neurological illness including, but not limited to, seizure disorders, frequent migraines (or on prophylaxis for same), multiple sclerosis, movement disorders, history of significant head trauma, or central nervous system (CNS) tumour) 31. The presence of other severe acute or chronic medical condition, psychiatric condition or laboratory abnormality that may increase the risk associated with participation or may interfere with interpretation of trial results. Please note: mild, stable chronic medical problems (e.g., Type 1 or 2 Diabetes Mellitus, HIV infection, glaucoma, hypothyroidism, hepatitis C, hepatic or renal disease, etc.) may be enrolled if Investigator and research psychiatrist agree that condition(s) would not: significantly increase risk of psilocybin administration, or be likely to produce significant symptoms during the study that could interfere with participation, or be confused with side effects of Investigational Product 32. Previous use of psilocybin or other psychedelic substance (except cannabis) on more than 5 occasions and/or use of the same within the past 5 years 33. Previous use of psilocybin, methylenedioxymethamphetamine (MDMA), ketamine (or substances reportedly containing psilocybin, MDMA, or ketamine) with therapeutic aim for current PTSD diagnosis 34. Has received Electroconvulsive Therapy (ECT) within 12 weeks of enrolment 35. Requires ongoing concomitant therapy with a psychiatric medication (unless deemed acceptable by the research psychiatrist) 36. Exposure to other investigational drug/device within 30 days of enrolment Medication exclusion criteria:

37. Over-the-counter products intended to affect mood/anxiety 38. Efavirenz 39. Lithium 40. "Rest-Category" Antidepressants (e.g., mirtazapine, trazodone, bupropion); Exception if ≤7.5mg mirtazapine, or ≤50mg trazodone as sleeping medication 41. Antipsychotics/Neuroleptics; Exception if ≤50mg quetiapine as sleeping medication 42. Stimulants 43. The following medications are permitted if the dose is hypnotic: selective serotonin reuptake inhibitor (SSRIs); Tricyclic antidepressants (TCAs); monoamine oxidase inhibitor (MAOIs) 44. The following medications are permitted if their use is unaltered during the study: Benzodiazepines "Z-drugs" (e.g., zolpidem); Anticonvulsants; Antihistamines 45. Medications which are permitted as determined case-by-case by research psychiatrist: non-psychiatric, but mind-altering medication (e.g., morphine, dexamethasone, etc.). 46. Not to be used 72hrs prior to psilocybin dosing session: Sildenafil (Viagra), tadalafil, or similar medications 47. Not to be used on dosing days and research psychiatrist discretion: Medical cannabis Risk exclusion criteria:

48. Current suicidal ideation/intent/action 49. Previous (within previous 6 months) suicidal ideation/intent/action 50. Current and previous deliberate self-harm

Gender Eligibility: All

Minimum Age: 18 Years

Maximum Age: 65 Years

Are Healthy Volunteers Accepted: No

Investigator Details

  • Lead Sponsor
    • Combat Stress
  • Collaborator
    • The Watson Trust
  • Provider of Information About this Clinical Study
    • Sponsor
  • Overall Official(s)
    • Prof. Dominic Murphy, Principal Investigator, Combat Stress
  • Overall Contact(s)
    • Prof. Dominic Murphy, 01372 587 017, dominic.murphy@combatstress.org.uk


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