Understanding the unwanted: a mixed-methods study on negative effects in an internet-based intervention for depression.

Internet-based interventions (IBIs) are effective for treating depression, but they can also lead to negative effects in some participants. There is no consensus on which specific characteristics of negative effects clinicians and researchers should focus on. Studies often combine distinct (sub)categories of negative effects, complicating interpretation. This study aimed to identify specific (sub)categories of negative effects related to depression and adherence and explore their predictors. In a sample of participants undergoing an IBI for depression (N = 1610; 61% female), 113 participants (7%) reported experiencing at least one negative effect. 110 participants qualitatively reported negative effects and were categorized into a framework consisting of two main categories (treatment-related vs. patient-related) of negative effects, divided into five subcategories (format, contact, implementation, symptoms, and insight). No differences in adherence were observed between any (sub)categories of negative effects; however, participants who reported treatment-related negative effects showed significantly lower symptom improvement than those reporting patient-related negative effects. No patient demographic characteristics predicted any negative effects. Differentiating treatment- and patient-related negative effects could enhance future research and intervention efforts.