Background Research suggests that many people enrolled in internet-based mental health interventions drop out without completing the program. We implemented an efficacious internet-based self-help tool for people affected by skin picking disorder, aiming to evaluate the impact of an early withdrawal in terms of self-efficacy and treatment motivation. Method We compared skin picking symptomatology (SPS), skin picking related self-efficacy (SP-SE) and motivation to change (SP-MtC) of 208 completers (52%) vs. 192 non-completers at 12-weeks post-assessment. Results The decrease in SPS was significantly greater for completers ( n  = 208; M  = 13.34; SD  = 4.72) compared to non-completers ( n  = 192; M  = 14.35; SD  = 4.94; Cohen’s d  = -.39) at post-assessment. SP-SE increased significantly more for program completers ( M  = 31.72; SD  = 6.50) compared to non-completers ( M  = 28.25; SD  = 6.30; Cohen’s d  = .66) at post-assessment. SP-MtC significantly decreased in both groups, with higher scores for completers across all assessments ( d pre-post  = .06). Conclusions Internet-based self-help tools can effectively reduce symptomatology. Participants, who do not complete the program, report lower treatment motivation and treatment-oriented self-efficacy, potentially reducing their help-seeking efforts. Treatment providers need to consider how to address these potentially adverse effects.