Cannabis use in youth is an important public health concern, especially as it often starts in young teens and in light of recent changes in Canadian legislation. The possible effects of cannabis (marijuana, weed, etc.) use on mental health are poorly understood. Cannabis is known to be associated with later psychosis. But what about more common mental health problems like depression, anxiety, and suicidal thoughts or attempts? Our first objective is to clarify the associations, if any, between cannabis use and later anxiety, depression, and suicidality, and whether they are causal or simply associative. Our second objective is to test the genetics-environment link – whether youth with pre-existing vulnerability to mental health problems are more at risk of depression, anxiety, and suicidality if they use cannabis, while the risk remains low for others. We will use data from the ALSPAC population-based longitudinal cohort. Participants had been asked at various times whether, how often, and when they started using, as well as questions on their mental health. Data will be analyzed by robust approaches to provide strong evidence in support (or not) of the links between cannabis use and later depression, anxiety, and suicidality. Clinically relevant patterns of cannabis use will be examined: a) ever used, b) age at onset, and c) intensity. Our findings will allow clinicians, researchers, and policymakers to develop better prevention and treatment programs to avoid mental health consequences of cannabis use in young people.