What if I tell you that there is no good evidence that marijuana does any good for treating glaucoma, anxiety, or Parkinson’s disease. Despite this, it is often prescribed to treat these aliments in states that have legalized marijuana. According to a large meta analysis of 79 studies published in Journal of the American Medical Association marijuana does nothing to improve these diseases.

However, this analysis did show that there is excellent evidence that marijuana is highly effective in controlling chronic pain. A JAMA review found a  “30% or greater improvement in pain with cannabinoid compared with placebo,” across the 79 studies it surveyed.

Now, couple this fact with the fact that there’s a body of research showing that painkiller abuse and overdose are lower in states with medical marijuana laws. There has been a long time assumption that when medical marijuana is available, pain patients choose marijuana over the prescription narcotics. But, until now that has just been an assumption.

A study from the Journal of Health Affairs clearly demonstrates the relationship between the use of medical marijuana to falling overdoses of opiates.

Ashley and W. David Bradford, researchers at the University of Georgia, evaluated the database of all prescription drugs paid for under Medicare Part D from 2010 to 2013. They found that, in the 17 states with a medical-marijuana law in p