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The data are from the cross-sectional 2012 Canadian Community Health Survey–Mental Health (CCHS–MH), which provides estimates of drug use, major mental disorders, and the use of mental health care services.^Note 14 The survey was developed by Statistics Canada in collaboration with Health Canada, the Public Health Agency of Canada, provincial health ministries, an expert advisory group, and academic experts.

The target population was the household population aged 15 or older in the 10 provinces. Data were collected from January through December 2012. The CCHS—MH excluded people living on Indian reserves and in other Aboriginal settlements, full-time members of the Canadian Forces, and the institutionalized population. Together, these exclusions represented about 3% of the target population. The response rate was 68.9%, yielding a sample of 25,113, representing 28.3 million Canadians.

Data from the 2002 Canadian Community Health Survey–Mental Health and Well-being, conducted from May 2002 through January 2003, were used for comparisons.^Note 17 The target population was comparable to that of the 2012 CCHS—MH. The response rate was 77%, yielding a sample of 36,984, representing 25.0 million Canadians.

Analyses were performed using SAS-callable SUDAAN v.11.0.1. Survey sampling weights were applied so that the analyses would be representative of the Canadian population. Variance estimation (95% confidence intervals) and significance testing were done using bootstrap replicate weights to account for the survey’s complex sampling design.

Past-year and lifetime marijuana use were based on responses to the following questions: “Have you ever used or tried marijuana or hashish?” and “Have you used it in the past 12 months?”  Past-year users were classified according to how frequently they used marijuana in the 12-months before they were interviewed: once, less than once a month, one to three times a month, weekly, or daily. Marijuana users (past-year and lifetime) were further subdivided into repeat users (one-time use excluded).  

The prevalence of past-year and lifetime marijuana use was examined by sex, age, province, residence (rural area versus population centre), household income quintile, and use of other illicit drugs, including cocaine, club drugs (ecstasy, ketamine), hallucinogens, heroin, inhalants/solvents, and selected psychotherapeutics (sedatives/tranquilizers, stimulants, and opioid analgesics) for non-medical purposes.

Population centres are continuously built-up areas with 1,000 or more inhabitants and a population density of 400 or more per square kilometre, based on current census population counts. Rural areas have fewer inhabitants and less population density.

Household income quintiles were derived by calculating the ratio of respondents’ total household income to the low-income cut-off corresponding to their household size and community. To minimize regional income differences, household income quintiles were estimated separately for each province and then pooled.

Although quality assurance measures were applied at each stage of data collection and processing^Note 18 and interviewers had extensive training, CCHS—MH data have several limitations. The information is self-reported and has not been verified. Social desirability and fear of punishment, both of which are potential sources of bias, may be especially relevant to this analysis. Social desirability is a tendency for respondents to modify their answers in an effort to construct a favourable image of themselves.^Note 19 Perceptions of what is "desirable" may differ depending on a respondent's age, sex and socioeconomic status; some respondents may under-report their drug use, and others may exaggerate it.  Research suggests that youth are particularly fearful of reprisal when reporting illegal activities,^Note 19 and therefore, marijuana use may be under-reported. As well, the extent of under-reporting may have differed in 2002 and in 2012. No information is available about intensity of use or the amount of active ingredients consumed. Analysis of marijuana confiscated by law enforcement suggests that the level of delta-9-tetrahydrocannabinol (THC), the active ingredient, has increased over time.^Note 20 The CCHS—MH did not collect information about age at initiation of marijuana use. Respondents whose past-year use varied considerably over the period were instructed to reference the period when use was highest. While not incorrect, this may overstate typical use. Finally, no information is available from the CCHS—MH about the use of marijuana for medical purposes.