The National Academy of Science recently released a review of the research on Cannabis - over 10,000 studies. It found SUBSTANTIAL EVIDENCE for:
Benefits
• For the treatment for chronic pain in adults (cannabis).
• For improving patient-reported multiple sclerosis spasticity symptoms (oral cannabinoids).
• Antiemetics in the treatment of chemotherapy-induced nausea and vomiting (oral cannabinoids).
Harms
• Worse respiratory symptoms and more frequent chronic bronchitis episodes (long-term cannabis smoking).
• Increased risk of motor vehicle crashes.
• Lower birth weight of the offspring.
• The development of schizophrenia or other psychoses, with the highest risk among the most frequent users.
• Initiating cannabis use at an earlier age is a risk factor for the development of problem cannabis use.
• Being male and smoking cigarettes are risk factors for the progression of cannabis use to problem cannabis use.

For full report, see:
http://nationalacademies.org/CannabisHealthEffects