Tobacco, cannabis, and other illicit drug use among Finnish adolescent twins: causal relationship or correlated liabilities?
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.
Huizink
Anja C
AC
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
Levälahti
Esko
E
Korhonen
Tellervo
T
Dick
Danielle M
DM
Pulkkinen
Lea
L
Rose
Richard J
RJ
Kaprio
Jaakko
J
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
D013287:Illicit Drugs