1. Addiction Research and Clinical Epidemiology Unit, Centre for Health and Society (chs), Institute of General Practice (ifam), Medical Faculty of the Heinrich-Heine-University , 40225 Duesseldorf , Germany
2. Department of Behavioural Science and Health, University College London , WC1E 7HB London , UK
3. Patient-Physician Communication Research Unit, Centre for Health and Society (chs), Institute of General Practice (ifam), Medical Faculty of the Heinrich-Heine-University , 40225 Duesseldorf , Germany