Substance use represents a primary global burden of disease for young people (1) as it has been associated with a wide range of adverse outcomes. These include physical health outcomes, such as infectious diseases and chronic bronchitis, social outcomes, including lower educational attainment and criminal activity, as well as increased substance dependence, risk of overdose, and mental health issues (1). Mental health symptoms and disorders have previously been associated with increased cannabis use in adolescence, including depression, suicidality, anxiety (2), and psychosis (3) . However, substance use and mental health are both complex, multifaceted issues that are likely to be affected by a number of risk factors (3).

One possible risk factor that may influence the relationship between substance use and mental health issues is socioeconomic status (SES), resulting in a greater burden of substance use on individuals of low SES. Research has identified that cannabis use is greater amongst individuals from low SES groups and may potentially be increasing over time (4). Whilst increased use amongst low SES individuals may result in worse mental health outcomes, it is also possible that, either alternatively or additionally, the social environment associated with reduced SES may exacerbate the harms associated with substance use, thus resulting in worse mental health outcomes. Increased harm related to other substances, such as tobacco, amongst low SES individuals has been observed previously (5), and therefore it may be plausible that a similar pattern is true for cannabis. This indicates that SES may be an important moderator of the relationship between adolescent cannabis use and mental health issues, including depression, anxiety, and psychotic experiences.

However, much of the current literature focuses on either substances other than cannabis or the impact of adult substance use and therefore further investigation is needed to understand the impacts of adolescent cannabis use . In addition, many studies have limited ability to draw causal inferences as a result of the high plausibility of reverse causation owing to the utilisation of cross-sectional data. This project aims to investigate the moderating role of SES in the relationship between adolescent cannabis use and mental health using longitudinal data from the ALSPAC cohort.