"These studies have generally assumed that when medical marijuana is available, pain patients are increasingly choosing pot over powerful and deadly prescription narcotics. But that's always been just an assumption. Now a new study, released in the journal Health Affairs, validates these findings by providing clear evidence of a missing link in the causal chain running from medical marijuana to falling overdoses...."
From "One striking chart shows why pharma companies are fighting legal marijuana" in The Washington Post.
ADDED: I'd like to see the analysis extended to states that have legalized marijuana beyond the medical/"medical" use. If we're talking about painkiller abuse, then it's not just about whether "medical marijuana" can replace doctor-prescribed drugs, but which drugs people are choosing on their own for whatever use they feel they have.