The key to using marijuana as medicine is to use low doses so as to minimize any danger. The difficulty with using marijuana as medicine is that the chemicals may inhibit drugs used to fight the malady as well. Anything systemic throughout a human body like nicotine, alcohol or caffeine can possibly inhibit real drugs trying to help. As such, marijuana may also be detrimental to one's therapy.
Many scientific studies have shown otherwise. Most statistical surveys and tests on the plant have reported no significant increase in cancer risk, decrease in lung function, or other dangerous health effects. While marijuana is not completely safe both physically and mentally, it is no more dangerous than most prescription drugs on the market, and arguably it is even less so.