Background: Increasing access to treatment via smartphone apps is an important topic in Social Anxiety Disorder (SAD). ‘Challenger’ is an app promoting exposure exercises in daily life. The present study evaluated the additional benefit of using the app as adjunct to Internet-based unguided self-help for SAD. In a second step, we also tested how the app and the self-help programme (SH) should best be combined.

Method: 209 patients diagnosed with SAD were randomly allocated to three groups. Group 1 received the app and the self-help programme for six weeks (parallel treatment), group 2 first received SH for six weeks and then the app for six weeks (sequential treatment). Group 3 was a wait-list group. Comparisons were made at week 7 evaluating the potential add-on effect of the app (SH plus app versus SH only) and at week 14 comparing the parallel to the sequential treatment. Participants filled in questionnaires prior, during, and post treatment, and at 4- and 12- months follow-up.

Results: Intention-to-treat analyses showed no significant effect of adding the app to Internet-based self-help. However, among participants actively using the app, adding Challenger to self-help resulted in significantly less social anxiety (d = 0.30). At week 14, decreases in social anxiety were large for both the parallel and the sequential group with no differences between the active groups (dwithin = 1.12–1.19). Changes were maintained throughout the follow-up period.

Conclusion: Results of the current study cautiously support the notion of adding a smartphone app to unguided self-help for SAD. Future studies should investigate how patients can be motivated to use the app more frequently.