Social Anxiety Disorder (SAD) is a mental disorder with high prevalence but low treatment accessibility. A way to facilitate care to these patients is through Internet based treatment. As is the case of most psychological treatments, much has been studied about positive effects but there is a gap regarding adverse effects. This study seeks to fill this gap in the case of an Internet delivered treatment. From a total of 127 participants who took part in a treatment that combined attention biased modification (ABM) and Internet based cognitive behavior therapy (iCBT), 21 (16.5%) presented adverse effects. Most of the adverse effects were detected after participants had received iCBT (13.4%), and these were mostly related to deterioration of symptoms (5.5%), negative wellbeing (3.9%) and emergence of new symptoms (1.6%). Perceived side effects after attention training were less common (4.7%). A thematic analysis showed also that the techniques used in treatment, the lack of time to complete treatment and becoming aware with the impairments of the disorder and its consequences could influence the emergence of adverse effects.