Background: The aim of this study was to investigate how general practitioners react when their cancer patients show interest in complementary medicine, and how their reaction is related to their knowledge in the field. Methods: We conducted semi-structured interviews with 10 German general practitioners. Interviewees came from 5 different federal states and varied in terms of urban/rural setting, single/joint practice, additional certifications, gender and length of professional experience. Interviews were electronically recorded, transcribed and then analysed using qualitative content analysis according to Mayring. Results: General practitioners feel largely responsible for providing information on complementary medicine to their cancer patients. However, uncertainty and a lack of knowledge concerning CAM lead mainly to reactive responses to patients' needs, and the general practitioners base their recommendations on personal experiences and attitudes. They wish to support their cancer patients and thus, in order to keep their patients' hopes up and maintain a trusting relationship, sometimes support complementary medicine, regardless of their own convictions. Conclusion: Although general practitioners see themselves as an important source of information on complementary medicine for their cancer patients, they also speak of their uncertainties and lack of knowledge. General practitioners would profit from training in complementary medicine enabling them to discuss this topic with their cancer patients in a proactive, open and honest manner.