Epidemiologic data indicate US young adult smokers use marijuana in greater amounts that their non-smoking peers. In 2009, 34.6% of smokers aged 18 to 25 reported past-month cannabis use compared with 8.9% of young adult nonsmokers . Depending on definitions of use, tobacco use increases the risk of cannabis use from 2 (e.g., past 30-day tobacco use is associated with past 30-day marijuana use ) to 52 times (e.g., having ever tried tobacco is associated with having ever tried marijuana ) in adolescents, and 3 to 6.4 times in adults [4–6].
Demographic differences have been observed in patterns of tobacco and marijuana involvement among young adults. Older youths [7, 8], males [6, 8–10], students in vocational schools , and those living in the Northeast and in small metropolitan areas [11, 12] are more likely to use tobacco or cannabis. There is a need to examine more detailed patterns of tobacco and marijuana use to understand the complex relationship between these two substances.
The internet is increasingly used in survey research of substance use [13, 14] with benefits over face-to-face interviews including broader reach; greater inclusion of low-incidence or “hidden” populations; rapid, convenient input by respondents; and reduced bias in response to sensitive, potentially stigmatizing topics including illicit substance use [15–18]. Young adults remain the age group most likely to use the internet (93% in a recent survey ), and they are less likely, compared to other age groups, to present to traditional research settings for studies of health behavior [20, 21]. Our prior research has demonstrated the reliability and validity of anonymous online surveys of young adult tobacco and cannabis use [22, 23].
Analyzing data from an anonymous online survey of young adult smokers with national coverage, the present study examined the prevalence of past-month marijuana use, frequency (days of use) among past-month marijuana users, and the frequency of co-using tobacco and marijuana. The large sample permitted analyses by gender, age, ethnicity, geographic region, urban/rural designation, student status, household income, daily smoking status, and by whether or not respondents resided in a state where marijuana is legal for medicinal use.