The general efficacy of cognitive behavior therapy for psychosis (CBTp) is well established. Although guidelines recommend that CBTp should be offered over a minimum of 16 sessions, the minimal number of sessions required to achieve significant changes in psychopathology has not been systematically investigated. Empirically informed knowledge of the minimal and optimal dose of CBTp is relevant in terms of dissemination and cost-effectiveness.

We approached the question of what constitutes an appropriate dose by investigating the dose (duration of CBTp) × response (symptomatic improvement) relationship for positive symptoms, negative symptoms and depression. Patients with psychotic disorders (n = 58) were assessed over the course of 45 sessions of CBTp in a clinical practice setting. At baseline and after session 5, 15, 25, and 45, general psychopathology, psychotic symptoms, symptom distress and coping were assessed with self-report questionnaires. Additionally, individually defined target symptoms and coping were assessed after each session.

Significant symptom improvement and reduction of symptom distress took place by session 15, and stayed fairly stable thereafter. The frequency of positive and negative symptoms reached a minimum by session 25.

Our findings support recommendations to provide CBTp over a minimum of 16 sessions and indicate that these recommendations are generalizable to clinical practice settings. However, the findings also imply that 25 sessions are the more appropriate dose. This study contributes to an empirically informed discussion on the minimal and optimal dose of CBTp. It also provides a basis for planning randomized trials comparing briefer and longer versions of CBTp.