Although cannabis has been linked to psychosis (see below) there is not such good evidence linking it to depression, anxiety and thoughts of suicide. Researchers from Cardiff and Bristol universities and the Karolinska Institute in Stockholm studied 50,087 men conscripted for military service in Sweden and compared the records from their army medicals with suicides recorded on the National Cause of Death Register over the next 33 years. There were 600 suicides among the group over the next 33 years with cannabis increasing the risk by 62%. However, the link between cannabis and suicide disappeared once the researchers took into account other factors. These included: having a low IQ, drinking heavily, having parents with mental-health problems, smoking heavily, using other drugs, having a psychiatric diagnosis when conscripted, having a 'high problematic behaviour score', 'poor psychological adjustment' and poor social relations.
Price, Ceri ... [et al] - Cannabis and suicide: longitudinal study British Journal of Psychiatry December 2009, 195(6), 492-497