title: Tobacco, cannabis, and other illicit drug use among Finnish adolescent twins: causal relationship or correlated liabilities?
abstract: OBJECTIVE
  Among Finnish adolescent twins, we compared (a) a model that describes a direct impact of liability to tobacco use on cannabis and other illicit drug use with (b) a model that included a shared underlying liability for these substances. Furthermore, the extent to which genetic and environmental influences contribute to the covariation between liabilities to use these substances was examined.
  
  
  METHODS
  Tobacco and illicit drug use were assessed at age 17.5 years. Twin data on 3,744 individuals were analyzed using standard biometrical methods. Two alternative multivariate models were fi t and compared with Mx, a statistical program for genetic model fitting.
  
  
  RESULTS
  The multivariate model, including a direct impact of the initiation of tobacco use on illicit drug use, provided the best fit to the data. In this model, the total variation in the initiation of illicit drugs was decomposed to genetic factors (32%), common environmental factors (20%), unique environmental factors (8%), and a component due to initiation of smoking (40%). Most variation in the progression of illicit drug use was the result of initiation of smoking and illicit drug use (83%).
  
  
  CONCLUSIONS
  Liability to initiate smoking directly affects illicit drug use in our best-fitting model. Our findings suggest that several common genetic influences may be related to tobacco use and illicit drugs but that a search for specific genes underlying illicit drug use is justifi ed as well. Such specific genes may hold a key to understanding biological vulnerabilities that lead to illicit drug use, which could aid in the development of targeted interventions.
authors.affiliation: Department of Education, Faculty of Behavioral and Social Sciences, University of Amsterdam, Nieuwe Prinsengracht 130, 1018 VZ, Amsterdam, The Netherlands. a.c.huizink@uva.nl
authors.forename: Anja C
authors.initials: AC
authors.lastname: Huizink
authors.forename: Esko
authors.initials: E
authors.lastname: Levälahti
authors.forename: Tellervo
authors.initials: T
authors.lastname: Korhonen
authors.forename: Danielle M
authors.initials: DM
authors.lastname: Dick
authors.forename: Lea
authors.initials: L
authors.lastname: Pulkkinen
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Rose
authors.forename: Jaakko
authors.initials: J
authors.lastname: Kaprio
mesh_terms: D000293:Adolescent; D000367:Age Factors; D015331:Cohort Studies; D005260:Female; D005387:Finland / Q000453:epidemiology; D006801:Humans; D013287:Illicit Drugs; D008297:Male; D002189:Marijuana Abuse / Q000453:epidemiology / Q000235:genetics; D008385:Marijuana Smoking / Q000453:epidemiology / Q000235:genetics; D012907:Smoking / Q000453:epidemiology* / Q000235:genetics*; D012931:Social Environment; D019966:Substance-Related Disorders / Q000453:epidemiology* / Q000235:genetics*; D014427:Twins / Q000235:genetics*; D055815:Young Adult
chemical_list: D013287:Illicit Drugs