Internet-based interventions are effective in the treatment of various mental disorders and have already been integrated in routine health care in some countries. Empirical data on potential negative effects of these interventions is lacking. This study investigated side effects in an Internet-based treatment for Social Anxiety Disorder (SAD).

A total of 133 individuals diagnosed with SAD took part in an 11-week guided treatment. Side effects were assessed as open formatted questions after week 2 and at post-treatment after week 11. Answers were independently rated by two coders. In addition, rates of deterioration and non-response were calculated for primary social anxiety and secondary outcome measures (depression and quality of life).

In total, 19 participants (14%) described unwanted negative events that they related to treatment. The emergence of new symptoms was the most commonly experienced side effect, followed by the deterioration of social anxiety symptoms and negative well-being. The large majority of the described side effects had a temporary but no enduring negative effect on participants' well-being. At post-treatment, none of the participants reported deterioration on social anxiety measures and 0–7% deteriorated on secondary outcome measures. Non-response was frequent with 32–50% for social anxiety measures and 57–90% for secondary outcomes at post-assessment.

Results suggest that a small proportion of participants in Internet-based interventions experiences negative effects during treatment. Information about potential side effects should be integrated in patient education in the practice of Internet-based treatments.