Age of onset of cannabis use and substance use problems: A systematic review of prospective studies.

Background: The association between the age of cannabis use (CU) onset and substance use (SU) problems has been extensively studied, yet findings remain inconsistent. Aims: This systematic review aimed to examine prospective studies on the association between age of CU onset and later SU problems, controlling for key individual, social, and SU-related risk factors. Methods: PsycINFO, Web of Science and PubMed were searched for studies published between January 2000 and December 2024. Studies were included if they: 1) were prospective; 2) measured CU onset during adolescence; 3) measured CU or SU problems after CU onset, 4) examined whole plant or phytocannabinoids. Studies were excluded if they exclusively focused on high-risk samples. Risk of bias was assessed using the Risk of Bias in Non-randomised Studies-of Interventions tool. The review was registered with PROSPERO, number CRD42022332092. Results: Sixteen studies met eligibility criteria. Earlier age of CU onset was associated with CU disorder (CUD) and CU negative consequences, with mixed findings for other SU problems (e.g., alcohol). CU frequency accounted for a significant portion of the risk for CU negative consequences, but the association with CUD remained independent of frequency. Only one study had low risk of bias, while seven had some concerns, and eight had a high or very high risk of bias. Conclusions: Findings suggest that early age of CU onset is a specific risk factor in the development of CUD, but not other SU problems. Prevention approaches should aim to delay the onset and reduce the frequency of CU among youth to reduce risk of the development of CUD.