The purpose of this study is to investigate the effect of Lu AG06466 after multiple doses of 30 milligrams (mg) in participants with PTSD.

The crossover study consists of two treatment periods of 15 days duration. On Day -1 or Day 1 of Treatment Period 1, eligible participants will be randomized (1:1) to a sequence of treatments (either Lu AG06466-placebo or placebo-Lu AG06466) with 15 participants are planned per sequence. Each treatment period will be separated by a washout period of ≥7 and ≤14 days.

Inclusion Criteria:

• The participant has PTSD, diagnosed according to Diagnostic and Statistical Manual Diploma in Social Medicine-5 (DSM-5), and confirmed by the Mini International Neuropsychiatric Interview (MINI).
• The participant has a Clinician-Administered PTSD Scale for DSM-5 (CAPS-5) total score ≥28 at screening and baseline.
• The participant has alterations in arousal and reactivity, confirmed on CAPS-5.
• The participant has ongoing sleep disturbances, confirmed on CAPS-5.
• The participant is willing to discontinue all prohibited medications during the study and to complete a washout of psychotropic medication during the washout period.
• The participant does not have any magnetic resonance imaging (MRI) contraindications.

Exclusion Criteria:

• The index traumatic event that led to development of PTSD took place >15 years or <6 months before screening.

Other inclusion and exclusion criteria may apply.