Mood disorders — There is substantial comorbidity between cannabis use/cannabis use disorder and mood disorders (depression, bipolar disorder). Secondary analyses of data from a representative sample of 43,093 community-based adults in the United States found that individuals with a lifetime mood disorder were two to three times more likely to have used cannabis during their lifetime compared with those without any psychiatric disorder [21] and to develop a cannabis use disorder after starting cannabis use [21,22]. Cross-sectional studies have found lifetime rates of cannabis use of approximately 70 percent and cannabis use disorder of approximately 30 percent among patients with bipolar disorder [23].
A systematic review of nine published community-based national epidemiologic surveys found a mean prevalence of 17 percent (range 10 to 30 percent) for current cannabis use disorder among respondents with bipolar disorder and a prevalence of 10 to 25 percent for bipolar disorder among respondents with current cannabis use disorder [24]. A systematic review by the same research group that included 78 published studies of inpatient and outpatient clinical populations found a 20 percent prevalence rate for cannabis use disorder among patients with bipolar disorder [25].
A systematic review of seven published prospective longitudinal cohort studies of adults with current mood disorder (five bipolar, two depressive) at baseline found that recent (prior six months) cannabis use was associated with higher levels of mood symptoms over time (2.5-month to five-year follow-up), compared with less intense or nonuse) [26].
Schizophrenia (nonaffective psychosis) — There is substantial comorbidity between cannabis use and schizophrenia; some experts believe that early cannabis use is a causal factor in developing schizophrenia. (See 'Psychotic disorders' below.)
Cross-sectional studies indicate that cannabis users have two- to threefold increased prevalence of schizophrenia compared with nonusers [27]. This association is stronger with earlier age of onset of use (eg, early adolescence), more intense cannabis use, and use of cannabis with high delta-9-tetrahydrocannabinol (THC) content and THC:cannabidiol ratio [28]. Secondary analyses of data from a representative sample of 43,093 community-living adults in the United States found that individuals with lifetime schizophrenia were two to three times more likely to have lifetime cannabis use than those without any psychiatric disorder [21] and to develop cannabis use disorder [21,22].
A systematic review of 53 published studies found that patients with schizophrenia-spectrum disorders had a 23.1 percent prevalence (range 4.5 to 81.1 percent) of cannabis use over the past 6 months and a 42.2 percent (range 19.2 to 89.1 percent) prevalence of lifetime use [29]. A systematic review of 35 published studies found that patients with schizophrenia-spectrum disorders had a 16.0 percent (8.6 to 28.6 percent interquartile range) prevalence of current cannabis use disorder and a 27.1 percent (12.2 to 38.5 percent interquartile range) prevalence of lifetime cannabis use disorder [30].
The increased prevalence of cannabis use by people with schizophrenia is not likely explained by a shared genetic liability. A cross-sectional study of 6931 adults in the Netherlands Twin Registry found that a polygenic risk score for schizophrenia (derived from a large genome-wide association meta-analysis) accounted for no more than 0.5 percent of the variance in several cannabis use phenotypes, including lifetime and regular use, frequency and quantity of use, and age at initiation of use [31].
A prospective, national, register-based, birth cohort study in Denmark that followed 41,470 people with schizophrenia born in 1955 or later found an increased risk of all-cause mortality in those with cannabis use disorder (hazard ratio 1.24, 95% CI 1.04-1.48, p = 0.0174) [32].