This RCT investigates whether Cognitive Processing Therapy, a first line treatment for PTSD, can be enhanced by the use of the odor molecule Hexadecenal (HEX). HEX is secreted from body odor and has been shown to promotes psychological well-being and positive social interaction. For the active group Hex will be diffused in the treatment room during all treatment sessions. For the control group a vehicle placebo molecule will be diffused. Both therapists and patients will be blind to study condition.

In this triple blind (patient, therapist, independent evaluator) RCT, 40 patients with PTSD receiving standard Cognitive Processing Therapy will be randomized in a 1:1 ratio to receive either odorless Hexadecanal molecules or odorless mineral oil molecules (Control) diffused in the treatment room at all therapy sessions. Inclusion criteria: age>18 years, PTSD diagnosis according to DSM-5-TR. Exclusion: Bipolar, psychotic, or neurological disorders, drug abuse, any parallel psychotherapy, anosmia. Primary outcome: pre-to-post treatment change in PTSD symptom severity measured by the PTSD check List for DSM-5 (PCL-5). Additional outcomes: Depression (PHQ-9), anxiety (GAD-7), Anger (DAR-5), therapeutic alliance (WAI-SF).

Inclusion Criteria:

• Diagnosis of PTSD according to DSM-5
• Age 18 years and over
• Eligible for CPT treatment at the National Center for Traumatic Stress and Resilience

Exclusion Criteria:

• Bipolar disorder, psychosis, neurological disorders
• Drug Addiction
• Parallel psychotherapy
• Congenital or acquired anosmia.

The randomization list is prepared in advance. Research assistants with no contact with the patients, therapists, or independent clinical assessors operate the odor dispensers according to the randomization schedule in the designated rooms where treatments take place.