The following is taken from an article 'The Mental Health Risks of Adolescent Cannabis Use' by Wayne Hall, Professor of Public Health Policy at the School of Population Health, University of Queensland, Herston, Australia. The full article can be viewed here.

Cannabis dependence

Cannabis users can become dependent on cannabis. The risk (around 10%) is lower than that for alcohol, nicotine, and opiates, but the earlier the age a young person begins to use cannabis, the higher the risk.

Cannabis and other illicit drug use

Regular users of cannabis are more likely to use heroin, cocaine, or other drugs, but the reasons for this remain unclear. Some of the relationship is attributable to the fact that young people who become regular cannabis users are more likely to use other illicit drugs for other reasons, and that they are in social environments that provide more opportunities to use these drugs. It is also possible that regular cannabis use produces changes in brain function that make the use of other drugs more attractive. The most likely explanation of the association between cannabis and the use of other illicit drugs probably involves a combination of these factors.

Cannabis and psychosis

As a rule of thumb, adolescents who use cannabis more than weekly probably increase their risk of experiencing psychotic symptoms and developing psychosis if they are vulnerable—if they have a family member with a psychosis or other mental disorder, or have already had unusual psychological experiences after using cannabis. This vulnerability may prove to be genetically mediated.

The full article can be viewed here - this is an information-dense article, so if you're interested in this topic do take time to read it.