"Longitudinal studies indicate that, in long-term, the higher use of cannabis is associated with an increased risk of developing bipolar disorder, and probably, major depression in subjects initially without affective disorder, but was not found increased risk of cannabis use among those initially only with mania or depression." http://www.ncbi.nlm.nih.gov/pubmed/20658056

the source is not very reliable(some obscure journal in portugese),