How to measure negative effects of psychological interventions

Negative effects of psychological interventions, such as deterioration, non-response, and novel symptoms, remain underexamined despite their ethical and clinical significance. This commentary reviews current methods for assessing such effects, highlighting the limitations of relying solely on symptom deterioration and the need for more direct patient-reported measures. Eight commonly used instruments are described and compared in terms of content, coverage, and psychometric properties. Methodological challenges, such as timing of assessment, measurement reactivity, and patients' reluctance to disclose harm, are also discussed. Recommendations are provided for improving the monitoring and reporting of negative effects in both clinical and research settings, with an emphasis on transparency, patient safety, and the inclusion of diverse populations and treatment modalities.