Cannabis smoking might contribute to the incidence and course of major depression, but it is less clear whether this relationship is particularly pernicious when exposure occurs during adolescence. This hypothesis is guided by prior evidence that adolescence is a particularly vulnerable period of neurodevelopment, and early cannabis exposure may have a lasting toxic effect on normal emotional development. The first study in this dissertation research investigates the degree to which early-onset cannabis smoking (before age 18 years) might predict the later onset of a sustained spell of depressed mood in adulthood (herein, a `depression spell'), as compared to never and later-onset cannabis users. The second study complements the findings therein and was motivated to inform on the extent to which depression spells were clinically relevant, with relevance operationally measured by functional impairment attributed to depression in an individual's occupational, social, and daily life. Data for these studies came from the U.S. National Surveys on Drug Use and Health (NSDUH), a program of annual cross-sectional surveys of large and nationally representative samples of community-dwelling U.S. residents aged 12 years or older.The third and fourth studies of this dissertation sought to contribute to our understanding of the epidemiology of cannabis use disorders via two relatively novel and understudied patterns of cannabis smoking. In the first, the tobacco-cannabis combination called `blunts', which has become an increasingly popular method of cannabis consumption in the U.S., was hypothesized to be associated with more cannabis problems (e.g., when blunt smokers are compared to their non-blunt cannabis smoking peers). Here also, the NSDUH epidemiological surveys made it possible to estimate the degree to which a history of blunt smoking is associated with the level of cannabis problems in a representative sample of recent cannabis users. For the final study in this dissertation research, it was hypothesized that cannabis users who rapidly transitioned from the first drug opportunity to using cannabis might have a greater risk of later cannabis problems. Here, for this study, the data are from the collaborative WHO World Mental Health Surveys (WHO-WMHS), with cross-national data from 14 countries that made it possible to look beyond the boundaries of the U.S. for novel epidemiological evidence on cannabis smoking.Results from these studies were informative. Early-onset cannabis smoking was associated with a later depression spell in adulthood: cases with an adult-onset depression spell were an estimated 70% more likely to have been exposed to using cannabis in adolescence, as compared to never users. However, the exposure odds ratio with respect to later-onset cannabis exposure was of similar magnitude, suggesting that early-onset cannabis exposure per se is less important than was hypothesized, and that the delay of cannabis onset until adulthood might not greatly affect the risk of a later depression spell. As for the issue of 'clinical relevance,' cases with recent depression spells suffered noteworthy functional impairment attributed to this mood disturbance, with greater impairment seen across at higher levels of cannabis problems. In specific, an estimated 25% (one in four) of the recent depression spell cases experienced severe to very severe functional impairment attributed to their depression. In the study of blunt smoking, as hypothesized, the level of cannabis problems was greater for blunt smokers and when there was more frequent recent blunt smoking, as compared to that experienced by cannabis users with little or no blunt smoking history. Findings from the cross-national epidemiological surveys were generally consistent with expectations -- namely, cannabis users who delayed their onset of cannabis smoking for a year or more after initially being offered the chance to try drugs were at a reduced chance of experiencing later cannabis-related problem outcomes.
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