This study is evaluating the safety, tolerability, and efficacy of methylone in adults with PTSD. The study will be conducted in two parts.

• Part A is open-label and will enroll up to 15 participants with PTSD
• Part B is randomized (1:1), double-blind, placebo-controlled and will enroll up to 64 participants with PTSD

Eligible participants will enter a 4-week Treatment Period where they will receive methylone once weekly for 4 weeks (4 treatment sessions). Following the Treatment Period, participants will enter a 6-week Follow-up Period which includes 3 reflection visits (Week 4, 5, and 6) and a final study visit at Week 10.

Inclusion Criteria:

• Meets the DSM-5 criteria for current moderate to severe PTSD diagnosis, with a symptom duration of at least 6 months
• CAPS-5 score of ≥35 at Screening.
• Failed at least one treatment for PTSD (either psychotherapy or pharmacological treatment).
• Proficient in reading and writing in local language sufficient to complete questionnaires.
• Free from any other clinically significant illness or disease

Exclusion Criteria:

• Primary diagnosis of any other DSM-5 disorder
• Body mass index (BMI) <18 kg/m2 or ≥40 kg/m2.
• Smokes an average of >10 cigarettes and/or e-cigarettes per day
• Uncontrolled hypertension at Screening
• Use of a psychedelic (e.g., LSD, psilocybin, DMT, mescaline), or entactogens such as MDMA, within 12 months of Screening.
• Use of an SSRI or other antidepressant within 8 weeks of screening.
• Current or previous history of clinically significant cardiovascular/cerebrovascular conditions.