This randomized controlled exploratory phase II trial will test the hypothesis that oral dronabinol improves nightmares (primary outcome) and other PTSD symptoms (secondary outcomes) to a greater extent than placebo over a ten week intervention phase in a parallel group design.

Inclusion Criteria:

1. Diagnosis of posttraumatic stress disorder (PTSD) according to DSM 5 with a 20 item CAPS-5 total score ≥ 26
2. At least two nightmares a week, an intensity score ≥ 2, with a CAPS-IV B2 (frequency and intensity for the last week) score ≥ 5
3. Men and women between 18 and 65 years of age
4. Written informed consent
5. The patient has the capacity to give consent (He/she is able to understand the nature and anticipated effects/side effects of the proposed medical intervention)
6. The patient is not breastfeeding
7. Women of child-bearing potential must have a negative urine or serum pregnancy test
8. All participants must use highly effective contraception
9. The patient received stable pharmacological medication for at least 4 weeks prior to study entry (any changes in medication dose or frequency of therapy must be answered with no)

Exclusion Criteria:

1. Lifetime cannabis use disorder
2. Current substance/alcohol use disorder (≤ 3 months);
3. Acute suicidality;
4. Psychotic disorder;
5. Bipolar disorder;
6. Current anorexia nervosa;
7. Current major depressive episodes and a MADRS score > 29;
8. Dementia;
9. Trauma-focused psychotherapy four weeks before the trial
10. Initiation of sleep medication 4 weeks prior screening or initiation of alpha adrenergic agents 4 weeks prior to screening
11. Acute or unstable medical illness.
12. Epilepsy
13. Relevant heart diseases
14. Known HIV- and/or active Hepatitis-B- or Hepatitis-C-infection
15. Current or past malignant illness
16. The patient is unwilling to consent to saving, processing and propagation of pseudonymized medical data for study reasons
17. Patients, who may be dependent on the sponsor, the investigator or the trial sites, have to be excluded from the trial
18. The patient is legally detained in an official institution
19. The patient does have a known allergy or contraindication against Dronabinol
20. The patient does have clinically significant abnormalities in 12-lead ECG
21. The patient does have clinically significant laboratory abnormalities
22. The patient did participate in other interventional trials during the 3 months before and at the time of this trial