• \>=21 years old

• Military Veteran

• Have given written informed consent

• Able to swallow pills

• Have a confirmed DSM-5 diagnosis of Post-traumatic Stress Disorder with symptom duration \>= 6 months

• Have a baseline CAPS-5 score of \>=28

• Be judged by study team clinicians to be at low acute risk for suicidality

• Concurrent psychotherapy is allowed if the type and frequency of the therapy has been stable for at least two months prior to screening and is expected to remain stable during participation in the study.

• Be medically stable as determined by screening for medical problems via a personal interview, a medical questionnaire, a physical examination, an electrocardiogram (ECG), and routine medical blood and urinalysis laboratory tests

• Agree to consume approximately the same amount of caffeine-containing beverage (e.g., coffee, tea) that he/she consumes on a usual morning, before arriving at the research unit on the mornings of drug session days. If the participant does not routinely consume caffeinated beverages, he/she must agree not to do so on session days.

• Agree to refrain from using any psychoactive drugs, including alcoholic beverages within 24 hours of each drug administration. The exceptions are caffeine and nicotine.

• Agree to refrain from using any caffeine or nicotine within 2 hours of dosing session start.

• Agree not to take any as needed (PRN) medications on the mornings of drug sessions without approval of the treatment team.

• Agree not to take sildenafil (Viagra®), tadalafil, or similar medications within 72 hours of each drug administration.

• Agree to stop taking 5HT2A antagonist medications at least 5 half-lives before dosing.

• Agree that for one week before each drug session, he/she will refrain from taking any nonprescription medication, nutritional supplement, or herbal supplement except when approved by the study investigators. Exceptions will be evaluated by the study investigators and will include acetaminophen, non-steroidal anti-inflammatory drugs, and common doses of vitamins and minerals.

• Have no lifetime use of serotonin 2A receptor (5-HT2A) agonist hallucinogens and/or MDMA at a greater dosage than a level typically defined as 'microdose.'

• Have at least a high school level of education or equivalent (e.g. GED).

• Weigh between 40kg- 120kg.

• Must provide a contact (relative, spouse, close friend or other support person) who is willing and able to be reached by the investigators in the event of a participant becoming suicidal or unreachable

• Agree to be released into the custody of a close friend or family member who has agreed to take charge of the study participant as a condition of leaving the testing facility, or agree to be accompanied home or to an accommodation by a member of the study team

• May have a history of or current Diabetes Mellitus (Type 2) if additional screening measures rule out underlying cardiovascular disease, if the condition is judged to be stable on effective management, and with approval by the Medical Monitor

• May have hypothyroidism if taking adequate and stable thyroid replacement medication

• May have a history of, or current, glaucoma if approval for study participation is received from an ophthalmologist