[1] Univ Oslo, SERAF, Norwegian Ctr Addict Res, N-0407 Oslo, Norway
[2] Norwegian Inst Publ Hlth, Div Epidemiol, Dept Pharmacoepidemiol, Oslo, Norway
[3] Kings Coll London, Natl Addict Ctr, London, England
[4] Municipal Ullensaker, Div Drug & Addict, Jessheim, Norway
[5] Oslo Univ Hosp, Div Mental Hlth & Addict, Oslo, Norway