Continued Cannabis Use Is Associated with Increased Incidence of Psychotic Symptoms

Prior investigations that demonstrated an association between cannabis use and psychosis had design limitations and could not confirm causality. In this prospective cohort study, investigators interviewed* a random sample of 1923 German adolescents and young adults at baseline (BL) and at 2 follow-up intervals (T2, 3.5 years; T3, 8.4 years) and examined results to determine the association between cannabis use and psychotic symptoms. Analyses were adjusted for potential confounders. Participants with baseline psychotic symptoms were excluded. Cannabis exposure was dichotomized as use ≥5 times over a lifetime at BL and use ≥5 times since the last interview at T2 and T3.

• The proportion of subjects reporting cannabis use at BL and T2 were 13% and 20%, respectively.

• The proportion of subjects reporting incident psychotic symptoms from BL to T2 and from T2 to T3 were 31% and 14%, respectively.

• The adjusted odds ratio (OR) of psychotic symptoms at T3 for persons with incident cannabis use at T2 was 1.9.

• The adjusted OR of psychotic symptoms at T2 and T3 among cannabis users was significant among persons who used cannabis at BL and T2 (2.2) but not significant among persons who used cannabis at BL but not T2 (2.1) or at T2 but not BL (1.4).

Comments:

This investigation’s strong design supports the temporal association between continued cannabis use and psychotic symptoms. However, use of “psychotic symptoms” rather than “psychotic disorder” as the outcome still leaves the relationship between cannabis use and mental illness diagnoses uncertain.

Hillary Kunins, MD, MPH, MS

Reference:

Kuepper R, van Os J, Lieb R, et al. Continued cannabis use and risk of incidence and persistence of psychotic symptoms: 1- year follow-up cohort study. BMJ. March 1, 2011;342:d738. doi: 10.1136/bmj.d738.