Medical Marijuana

The issue of Medical Marijuana is a slam dunk. There is no reason that a drug that has proven benefit shouldn't be prescribed to patients by their physicians. It is ludicrous to talk about the dangers of marijuana and the potential for abuse while doctors are capable of prescribing opiates (that derive from the same plant as heroin). Why is it that doctors can't prescribe a pain killer with little potential for addiction (marijuana) when they can prescribe several pain killers (morphine, codeine, oxycodon, oxycontin, etc) that have a moderate to high risk for addiction?