But the study left one big question unanswered: Since the study's authors — the University of Georgia's Ashley Bradford and W. David Bradford — only looked at Medicare data, they couldn't say for sure whether the findings held for younger patients too.

Now, we're getting more answers. This week, the Bradfords are back with a new study applying the same analysis to prescriptions under Medicaid, which covers low-income people of all ages. The results largely validate their previous work: Medicaid prescriptions for certain drugs fell significantly in states that adopted a medical marijuana law.

Specifically, anti-nausea drug prescriptions fell by 17 percent. Antidepressant prescriptions fell 13 percent, while prescriptions for seizure and psychosis drugs fell 12 percent.

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Not everyone who has access to medical marijuana opts for it, obviously. But enough do to make a significant dent in the prescription numbers. “Patients and physicians in the community are reacting to the availability of medical marijuana as if it were medicine,” the Bradfords conclude.

Perhaps most significantly from a public health standpoint, prescriptions for painkillers fell by 11 percent. Opiate painkillers are behind much of the current drug overdose epidemic.

Numerous studies have found that opiate abuse and overdose rates fell in states with medical marijuana laws. The Bradfords' research describes the mechanism by which that could happen: the introduction of medical marijuana laws coincides with a drop in painkiller prescriptions.

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The Bradfords' data only include prescriptions made under Medicare and Medicaid, but given the totality of their evidence it seems reasonable to assume that similar patterns hold true for patients on private insurance plans.

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In the current budgetary environment, no analysis of health care is complete without a discussion of costs. The Bradfords estimate that because of the drops in prescribing rates, a nationwide medical marijuana program would save taxpayers about $1.1 billion on Medicaid prescriptions annually. That's on top of the half a billion in Medicare savings the Bradfords estimated last year.

Those costs don't evaporate into thin air, of course: they would simply be shifted over to seniors and low-income people who would be purchasing medical marijuana outside of their insurance programs.

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There's also the open question of how smart it is from a personal health perspective to be self-medicating with pot: “it is plausible that forgoing medications with known safety, efficacy, and dosing profiles in favor of marijuana could be harmful under some circumstances,” the Bradfords warn.

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Still, the Bradfords' research makes the gulf between our current medical understanding of marijuana and federal policy around it even wider. Last summer the DEA affirmed yet again marijuana's status in Schedule 1 of the Controlled Substances Act, a category reserved for the most dangerous, addictive drugs that have no medical applications.