This study found that cannabinoid receptor agonists (such as marijuana) seem to be potent agents to help inhibit prostate cancer.
   
According to the study “Our results suggest that treatment of androgen-responsive human prostate carcinoma LNCaP cells resulted in a decrease in intracellular and secreted levels of PSA, with concomitant inhibition of androgen receptor, cell growth, and induction of apoptosis. We conclude that cannabinoids should be considered as agents for the management of prostate cancer
  
This may end up being useful as a preventative measure for people who are at risk for prostate cancer, though our society has a way to go to get over the negative stigma that Marijuana has.
   
A word of caution. Keep in mind that there are studies showing that, while safe in general, there are some people who can have adverse psychological effects from Marijuana. If there is a personal or family history of psychosis (schizophrenia, etc.), then it is probably best to avoid marijuana use since there is some evidence it could trigger it if you are already “predisposed”. There is little to no evidence that moderate doses of marijuana can induce psychosis in individuals that are not predisposed. However, high doses seem to be able to do that…. more information below.
   
Cancer Research 65, 1635-1641, March 1, 2005
   
Drug Alcohol Rev. 2004 Dec;23(4):433-43. Cannabis use and psychotic disorders: an update.
Hall W, Degenhardt L, Teesson M.
   
Office of Public Policy and Ethics, Institute for Molecular Bioscience University of Queensland Australia. w.hall@imb.uq.edu.au
   
The following is an abstract of the above study looking at cannabis use and psychosis
   
This paper evaluates three hypotheses about the relationship between cannabis use and psychosis in the light of recent evidence from prospective epidemiological studies. These are that: (1) cannabis use causes a psychotic disorder that would not have occurred in the absence of cannabis use; (2) that cannabis use may precipitate schizophrenia or exacerbate its symptoms; and (3) that cannabis use may exacerbate the symptoms of psychosis. There is limited support for the first hypothesis. As a consequence of recent prospective studies, there is now stronger support for the second hypothesis. Four recent prospective studies in three countries have found relationships between the frequency with which cannabis had been used and the risk of receiving a diagnosis of schizophrenia or of reporting psychotic symptoms. These relationships are stronger in people with a history of psychotic symptoms and they have persisted after adjustment for potentially confounding variables. The absence of any change in the incidence of schizophrenia during the three decades in which cannabis use in Australia has increased makes it unlikely that cannabis use can produce psychoses that would not have occurred in its absence. It seems more likely that cannabis use can precipitate schizophrenia in vulnerable individuals. There is also reasonable evidence for the third hypothesis that cannabis use exacerbates psychosis.
PMID: 15763748 [PubMed – indexed for MEDLINE]