Cannabis use : understanding other illicit drug use, drug-related morbidity and dependence

Amid increased prevalence globally of both cannabis use, and cannabis use disorder (CUD), changes of the legal status of use, as well as increased cannabis potency, it is important to increase the understanding about the health effects from using this psychoactive substance. Improved understanding will provide better prerequisites when shaping policies and healthcare systems targeting affected individuals. Thus, the aim of this thesis was to increase understanding of cannabis use, other illicit drug use, drug-related morbidity, and dependence. The studies were based on different sources of information (surveys, interviews, register linkages) and a variety of methodological approaches (longitudinal, cross-sectional, and cluster designs). Study 1 examined cannabis use in relation to other illicit drug use and drug use disorders. Survey data was used, comprising adults aged 20-64 years from the general population in Stockholm Region (n = 9 733). The results showed that cannabis use did not seem to act as an independent risk factor for later drug use disorders, although cannabis use did increase the risk of other illicit drug use at three-year follow-up. Of the included covariates, alcohol consumption attenuated the associations the most. Study 2 assessed the relationship across time between cannabis use and anxiety as well as depression. Interview data was used, comprising women born 1955-1993 from the general population in Gothenburg municipality (n = 1 100). The results showed that cannabis using women born in later years were at higher risk of depression and anxiety. The results from the interaction analyses indicated that period of cannabis use increased the risk of depression. Childhood factors (unsafe upbringing and family tensions) attenuated the associations. Study 3 explored the socioeconomic characteristics and psychiatric comorbidity of individuals with CUD compared to those without. Register data was used to derive the study population, which comprised all individuals born 1970-2000, and registered as living in Sweden sometime between 1990 and 2016 (n = 3 307 759). Four clusters were identified, two of which showed slightly higher proportion of CUD. One of those clusters was characterized by young men with low income and other substance use disorders, and the other cluster was characterized by young women with high income and behavioral disorders. Study 4 examined CUD readmissions and the influence of socioeconomic factors and psychiatric comorbidity on the risk of being readmitted to healthcare for a CUD diagnosis. Register data was used to derive the study population, which comprised individuals with a CUD diagnosis born 1950-1999, and registered as living in Sweden sometime between 2001 and 2016 (n = 12 143). The results showed that CUD visits mainly took place in the outpatient care (~80%), and that low education, schizophrenia and psychotic disorders, personality disorders, or mood disorders increased the risk of CUD readmission the most. Individuals aged 18-35 years were at higher risk of readmission. In conclusion, the findings in this thesis show associations between cannabis use, other illicit drug use and psychiatric disorders. Those reporting cannabis use or are diagnosed with CUD are primarily younger individuals, and mainly men, who also suffer from other substance use disorders. On the other hand, women who use cannabis or are diagnosed with CUD are often diagnosed with mood- related disorders, neurotic and stress-related disorders, and behavioral disorders. Risk of being readmitted to healthcare for a CUD diagnosis was highest among young individuals, those with only primary education, schizophrenia and other psychotic disorders, mood-related disorders, or personality disorders. The implications of these findings are of relevance to healthcare, as they inform on the complex healthcare needs of individuals with CUD and their psychiatric comorbidity/multimorbidity – which in turn may affect the risk of readmission. Young individuals are central with regards to cannabis use and CUD. Additionally, since women, to a larger extent than men, visit healthcare for a variety of mental health problems, their possible substance use disorders may be overlooked, hence particular attention should be given to these women