Side effects (SEs) are negative reactions to an appropriately delivered treatment, which must be discriminated from unwanted events (UEs) or consequences of inadequate treatment. One hundred CBT therapists were interviewed for UEs and SEs in one of their current outpatients. Therapists reported 372 UEs in 98 patients and SEs in 43 patients. Most frequent were "negative wellbeing/distress" (27% of patients), "worsening of symptoms" (9%), "strains in family relations" (6%); 21% of patients suffered from severe or very severe and 5% from persistent SEs. SEs are unavoidable and frequent also in well-delivered CBT. They include both symptoms and the impairment of social life. Knowledge about the side effect profile can improve early recognition of SEs, safeguard patients, and enhance therapy outcome.