Overall Psychological Safety of Cannabis: Schizophrenia and Psychosis
"A review of the literature suggests that the majority of cannabis users, who use the drug occasionally rather than on a daily basis, will not suffer any lasting physical or mental harm. Conversely, as with other 'recreational' drugs, there will be some who suffer adverse consequences from their use of cannabis. Some individuals who have psychotic thought tendencies might risk precipitating psychotic illness. Those who consume large doses of the drug on a regular basis are likely to have lower educational achievement and lower income, and may suffer physical damage to the airways. They also run a significant risk of becoming dependent upon continuing use of the drug. There is little evidence, however, that these adverse effects persist after drug use stops or that any direct cause and effect relationships are involved."
Iversen, Leslie L., PhD, FRS, "Long-Term Effects of Exposure to Cannabis," Current Opinion in Pharmacology, Feb. 2005, Vol. 5, No. 1, p. 71.