title: Patterns of use, sequence of onsets and correlates of tobacco and cannabis.
abstract: BACKGROUND
  While most individuals initiate their use of tobacco prior to onset of cannabis use, recent reports have identified a smaller subset of youth who report onset of cannabis use prior to tobacco use. In this study, we characterize patterns of cannabis and tobacco use (tobacco but not cannabis, cannabis but not tobacco or both) and compare the factors associated with onset of tobacco before cannabis and cannabis before tobacco.
  
  
  METHODS
  Data on 1812 offspring aged 12-32 years, drawn from two related offspring of Vietnam Era twin studies, were used. Individuals were divided into tobacco but not cannabis (T), cannabis but not tobacco (C) and users of both substances (CT). Those who used both could be further classified by the timing of onset of tobacco and cannabis use. Multinomial logistic regression was used to characterize the groups using socio-demographic and psychiatric covariates. Furthermore, data on parental smoking and drug use was used to identify whether certain groups represented greater genetic or environmental vulnerability.
  
  
  RESULTS
  22% (N=398) reported T, 3% (N=55) reported C and 44% reported CT (N=801). Of the 801 CT individuals, 72.8% (N=583), 9.9% (N=77) and 17.3% (N=139) reported onset of tobacco before cannabis, cannabis before tobacco and onsets at the same age. C users were as likely as CT users to report peer drug use and psychopathology, such as conduct problems while CT was associated with increased tobacco use relative to T. Onset of tobacco prior to cannabis, when compared onset of cannabis before tobacco or reporting initiation at the same age was associated with greater cigarettes smoked per day, however no distinct factors distinguished the group with onset of cannabis before tobacco from those with initiation at the same age.
  
  
  CONCLUSIONS
  A small subset of individuals report cannabis without tobacco use. Of those who use both cannabis and tobacco, a small group report cannabis use prior to tobacco use. Follow-up analyses that chart the trajectories of these individuals will be required to delineate their course of substance involvement.
authors.affiliation: Washington University School of Medicine, Dept. of Psychiatry & the Midwest Alcoholism Research Center, Saint Louis, MO 63110, USA. arpana@wustl.edu
authors.forename: Arpana
authors.initials: A
authors.lastname: Agrawal
authors.forename: Jeffrey F
authors.initials: JF
authors.lastname: Scherrer
authors.forename: Michael T
authors.initials: MT
authors.lastname: Lynskey
authors.forename: Carolyn E
authors.initials: CE
authors.lastname: Sartor
authors.forename: Julia D
authors.initials: JD
authors.lastname: Grant
authors.forename: Jon Randolph
authors.initials: JR
authors.lastname: Haber
authors.forename: Pamela A F
authors.initials: PA
authors.lastname: Madden
authors.forename: Theodore
authors.initials: T
authors.lastname: Jacob
authors.forename: Kathleen K
authors.initials: KK
authors.lastname: Bucholz
authors.forename: Hong
authors.initials: H
authors.lastname: Xian
mesh_terms: D000293:Adolescent; D000328:Adult; D017668:Age of Onset; D002648:Child; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D008297:Male; D008385:Marijuana Smoking / Q000453:epidemiology*; D012189:Retrospective Studies; D012307:Risk Factors; D057566:Self Report; D012907:Smoking / Q000453:epidemiology*; D014481:United States / Q000453:epidemiology; D055815:Young Adult