title: Tobacco smoking and cannabis use in a longitudinal birth cohort: evidence of reciprocal causal relationships.
abstract: BACKGROUND
  There is evidence of associations between tobacco and cannabis use that are consistent with both a classical stepping-stone scenario that posits the transition from tobacco use to cannabis use ('gateway' effect of tobacco) and with the reverse process leading from cannabis use to tobacco abuse ('reverse gateway' effect of cannabis). The evidence of direct causal relationships between the two disorders is still missing.
  
  
  METHODS
  We analysed data from the Christchurch Health and Development Study (CHDS) longitudinal birth cohort using advanced statistical modelling to control for fixed sources of confounding and to explore causal pathways. The data were analysed using both: (a) conditional fixed effects logistic regression modelling; and (b) a systematic structural equation modelling approach previously developed to investigate psychiatric co-morbidities in the same cohort.
  
  
  RESULTS
  We found significant (p<0.05) associations between the extent of cannabis use and tobacco smoking and vice versa, after controlling for non-observed fixed confounding factors and for a number of time-dynamic covariate factors (major depression, alcohol use disorder, anxiety disorder, stressful life events, deviant peer affiliations). Furthermore, increasing levels of tobacco smoking were associated with increasing cannabis use (p=0.02) and vice versa (p<0.001) over time.
  
  
  CONCLUSIONS
  Our results lend support to the notion of both of 'gateway' and 'reverse gateway' effects. That is, the association between tobacco and cannabis use arises from a reciprocal feedback loop involving simultaneous causation between tobacco use disorder and cannabis use disorder.
authors.affiliation: Sussex Addiction Research and Intervention Centre (SARIC), School of Psychology, University of Sussex, Brighton, United Kingdom.
authors.forename: Aldo
authors.initials: A
authors.lastname: Badiani
authors.affiliation: Christchurch Health and Development Study, University of Otago, Christchurch School of Medicine and Health Sciences, PO Box 4345, Christchurch, New Zealand. Electronic address: joseph.boden@otago.ac.nz.
authors.forename: Joseph M
authors.initials: JM
authors.lastname: Boden
authors.affiliation: Sussex Addiction Research and Intervention Centre (SARIC), School of Psychology, University of Sussex, Brighton, United Kingdom.
authors.forename: Silvana
authors.initials: S
authors.lastname: De Pirro
authors.affiliation: Christchurch Health and Development Study, University of Otago, Christchurch School of Medicine and Health Sciences, PO Box 4345, Christchurch, New Zealand.
authors.forename: David M
authors.initials: DM
authors.lastname: Fergusson
authors.affiliation: Christchurch Health and Development Study, University of Otago, Christchurch School of Medicine and Health Sciences, PO Box 4345, Christchurch, New Zealand.
authors.forename: L John
authors.initials: LJ
authors.lastname: Horwood
authors.affiliation: Rudd Centre for Adoption Research and Practice, School of Psychology, University of Sussex, and Institute of Genetic, Neurobiological and Social Foundations of Child Development, Tomsk State University, Brighton, United Kingdom.
authors.forename: Gordon T
authors.initials: GT
authors.lastname: Harold
mesh_terms: D000293:Adolescent; D000328:Adult; D019973:Alcohol-Related Disorders / Q000453:epidemiology*; D001008:Anxiety Disorders / Q000453:epidemiology*; D015897:Comorbidity; D003865:Major Depressive Disorder / Q000453:epidemiology*; D005260:Female; D006801:Humans; D008016:Life Change Events; D008137:Longitudinal Studies; D008297:Male; D008385:Marijuana Smoking / Q000453:epidemiology*; D015233:Models, Statistical; D009520:New Zealand / Q000453:epidemiology; D010379:Peer Group; D012907:Smoking / Q000453:epidemiology*; D055815:Young Adult
keywords: Cannabis; Longitudinal study; Structural equation Modelling; Tobacco