[1] Oslo Univ Hosp, Dept Outpatient Addict Recovery, N-0424 Oslo, Norway
[2] Univ Oslo, Norwegian Ctr Addict Res SERAF, Oslo, Norway
[3] Norwegian Inst Publ Hlth, Dept Pharmacoepidemiol, Oslo, Norway
[4] Oslo Univ Hosp, Div Mental Hlth andAddict, N-0424 Oslo, Norway
[5] Univ Oslo, Inst Clin Med, Oslo, Norway