The U.S. Food and Drug Administration's reviewers have raised efficacy concerns over Otsuka Pharma's combination drug for adults with PTSD, citing inconsistent trial results and a modest treatment effect that may not be clinically meaningful.

The FDA staff reviewers' assessment on Wednesday comes ahead of a meeting of its independent experts on Friday, who will make recommendations on whether the regulator should approve the use of the drug, brexpiprazole.

The Japanese drugmaker is seeking approval for brexpiprazole in combination with Viatris' Zoloft for the treatment of adults with PTSD.

The drug failed to demonstrate substantial evidence of effectiveness for PTSD when added to sertraline, due to concerns about study design and inconsistent study results, the reviewers said in briefing documents released on the FDA's website.

PTSD is a mental health condition that can develop after a traumatic event, causing symptoms including flashbacks, nightmares and severe anxiety.

It is estimated to affect about 4% of U.S. adults, according to the American Psychiatric Association.