We found that CUD diagnoses increased over time, especially among those born in 1990 and later. About 78% of individuals diagnosed with CUD were male and 80% had an additional psychiatric disorder. Nearly half of the individuals with CUD had attained primary education only, and one-third belonged to the lowest income group, compared with 16% and 24% respectively among individuals without CUD.

We observed a large increase in number of CUD diagnoses in our youngest cohort (born 1995–2000) between the years 2011 and 2016. Their age range during this period was 11–20 years; however, the majority received their first CUD diagnosis between ages 16 and 18 years (not shown). Considering that CUD onset has been shown to occur within the first year of cannabis use, with younger users showing an even higher risk for CUD compared with their older counterparts [20], we would expect an increase in cannabis use approximately during the same time period. The availability of cannabis has increased in Sweden during the past 10 years, although cannabis use has been quite stable during the last 20 years, with self-reported past-month use at about 2–4% among young people (16–29 years) [21]. There has, however, been a slight increase in frequent cannabis use among 16–19-year olds, where those reporting cannabis use have increased their use from an average of four times in 1989 to 13 times in 2016 [22]. Also, an increased cannabis potency in recent years might imply higher rates of harmful effects requiring healthcare. Overall, the increase in CUD diagnoses despite the stability in use is in line with results from a recent Norwegian study [23].

We identified four clusters, where two of the clusters (1 and 4) showed higher proportions of CUD. Cluster 1 included mostly men, a majority born in 1990–1994, in the lower income groups, and with a high proportion of other substance-related diagnoses. Cluster 4 included mostly women, born in 1995–2000, in the highest income group, and with a high proportion of behavioural disorders. The most notable finding, and somewhat in contrast to what we expected, was perhaps the lack of a clear CUD cluster.

Our findings correspond to those of previous studies showing associations between CUD and several psychiatric disorders [5,7,23]. For example, an Australian study showed that seven out of 10 people with a CUD also had another psychiatric disorder [24]. Our findings are also in line with those showing CUD to be more common among men [6] and the younger population (e.g. 18–29 years) [8].

Clusters 1 and 4 had the highest proportions of CUD. The clusters were similar with regard to birth year, educational level and levels of mood-related and neurotic and stress-related disorders. However, they also differed, as cluster 1 included mostly men with low income and cluster 4 included mostly women with high income. Several explanations for their similarities and differences are possible. It may be that the younger population is more sensitive to cannabis use [25], which would put them at a higher risk of developing CUD [8,9]. It may also be that the level of cannabis use is higher in younger age groups [3] and/or that the cannabis used in later years (inevitably relevant for the younger birth cohorts) is more potent with higher concentration of the psychoactive substance ∆-9-tetrahydrocannabinol [25]. Concerning the income differences between these two clusters, some previous studies have shown less affluent adolescents to be at higher risk of frequent cannabis use [26]. High levels of cannabis use during adolescence have also been shown to increase the risk for low income later in life [27]. In contrast, high household income during adolescence has also been associated with high rates of cannabis use [28]. However, none of these studies has considered the possible sex differences with regard to income and CUD, which our study seems to indicate. The similarity in educational level between these two clusters is probably related to the age distribution, especially among those born 1995–2000 who were not old enough to have completed secondary education during our study period.

Clusters 2 and 3 were similar in most regards, including the distribution of sex and psychiatric disorders with high proportions of anxiety and depression. These clusters encompassed the older birth cohorts and essentially all income groups. Official statistics show increased rates of anxiety and depression mainly among women in Sweden during recent years [29]. Interestingly, the composition of cluster 4 was different, with women born in 1995–2000, from affluent circumstances with a higher proportion of CUD and behavioural disorders. Yet, the association between behavioural disorders, such as attention deficit hyperactivity disorder (ADHD), and substance use disorders is well-known and a recent study reported women with ADHD to be almost three times more likely to have a drug use disorder compared with men with ADHD [30]. Still, women may be less likely to be identified as problematic cannabis users within healthcare, since men with CUD to a large extent also exhibit other substance-related disorders, while women instead are diagnosed with mood, neurotic or behavioural disorders. This suggests that more attention should be directed towards women with CUD as they are likely dealing with varied psychiatric disorders.