1. Norwegian Centre for Addiction Research, Institute of Clinical Medicine, University of Oslo , Pb 1039 Blindern , 0316 Oslo , Norway
2. Norwegian Centre for Addiction Research, Institute of Clinical Medicine, University of Oslo , Oslo , Norway
3. Department of Oncology , Rigshospitalet Copenhagen University Hospital , Copenhagen , Denmark
4. Department of Clinical Medicine, Faculty of Health and Medical Sciences , University of Copenhagen , Copenhagen , Denmark
5. Department of Medicine, Karolinska Institutet , Stockholm , Sweden
6. Department of Mental Disorders, Norwegian Institute of Public Health , Oslo , Norway