Frequency and reasons for switching/stopping use of oral contraceptives. Results of the German Cohort Study on Women Health. Use of oral contraceptives is the safest and most commonly used method of contraception. The frequency of use underscores the importance of research on benefits, risks, and acceptability of long-term use. - An interim analysis of the German Cohort Study on Women's Health (aged 15-65 years) was performed to analyse frequency and reason of switching or stop of OC-use. We got for this analysis over 10 000 women with almost 100 000 years of observation from first to last (or current) OC use. - The prevalence of ever use of OCs was 85.5 % in our cohort. Switching or stop of OC use was observed in 93 % of the women. The frequency of switching increased with age and duration of use. Social parameter such as education, employment status, and living in the old or the new Federal States were not materially associated with the frequency of switching or stopping OC use. Satisfaction with general health, psychic stability, and sexual life was slightly associated with the frequency of switching: The better satisfaction the lower frequency of switching, but the sequence of cause and consequence is unclear. The most frequent categories of reasons for switching were observed side effects and personal reasons. Desire to get pregnant was the number one single reason, followed by headache/migraine, and bleeding problems. The frequency of more than 50 single reasons is listed. - The frequency and the knowledge of reason for switching or stopping OC use underlines the need for a good collaboration between physician and treated woman. However, also research on long-term compliance is required and the development of OCs with even less side effects.