In a longitudinal analysis of twins, those who used cannabis more often than their identical twin during adolescence had worse socioeconomic outcomes in early adulthood, according to researchers.
Previous studies have demonstrated an association between cannabis use and cognitive impairment and lower socioeconomic attainment, Jonathan D. Schaefer, PhD, of the Institute of Child Development at the University of Minnesota, and colleagues wrote in the Proceedings of the National Academy of Sciences. However, the causal link is unclear, they added.
Schaefer and colleagues analyzed data from three longitudinal studies involving 3,762 twins, including 2,410 monozygotic twins. The researchers used a continuous adolescent cannabis use index and compared psychiatric, cognitive and socioeconomic outcomes — including educational attainment, occupational status and income — between twins through early adulthood.
They wrote that 29% of the study sample reported “at least some” adolescent cannabis use, with “11% meeting criteria for an adolescent cannabis use disorder.”
Schaefer said that adolescents who used more cannabis than their identical twins “tended to experience poorer academic functioning in middle and high school.” In early adulthood, a within-pair analysis revealed a significant association between cannabis use and reduced educational attainment for these participants (OR = –0.15; 95% CI, –.23 to –.06).
Participants who used more cannabis than their twins in adolescence also had “multiple negative psychiatric and socioeconomic outcomes,” the researchers wrote, including a within-pair OR estimate of 1.58 (95% CI, .98-2.54) for anxiety disorder.
“However, we did not find evidence supporting the existence of a causal relationship between adolescent cannabis use and either adult mental health or cognitive ability; these associations seemed more likely to arise because of other shared risk factors,” Schaefer said. “Considered together, these findings suggest that the effects of adolescent cannabis use on adult functioning may be limited to short-term drug effects with longer-term impacts (eg, perhaps cannabis use causes temporary decreases in motivation, which leads to poorer grades, which leads to lower educational attainment), rather than dramatic effects on brain development with long-term impacts on future functioning.”
Nonetheless, there are still concerns regarding mental health and cannabis, Schaefer said.
“It has been repeatedly shown that the acute effects of cannabis can temporarily worsen at least some forms of existing mental health problems (e.g., psychotic disorders), so our results should not be interpreted as suggesting cannabis use is entirely harmless from a mental health perspective,” he added.
Other research suggests that early cannabis use often occurs in the presence of early-developing mental health issues and family or related risk factors, according to Schaefer.
“Our results support the perspective that actions or treatments aimed solely at reducing teen cannabis use without addressing these broader issues are generally unlikely to produce long-term positive effects,” he said. “Therefore, connecting cannabis-using teens and their families with mental health professionals, including individual and family therapists, who can assess these and other problems and work with the family to address them, is almost never a bad first step.”
Schaefer and colleagues are continuing to follow the study participants. They are also collaborating with researchers on a twin study in Colorado, “which will offer opportunities to consider important questions about cannabis use in states with and without legalized recreational marijuana,” Schaefer said.
