OxyContin pills (Toby Talbot/AP)

Sen. Ron Johnson (R-Wis.) convened a hearing last week to explore the theory that the Affordable Care Act’s expansion of Medicaid caused the nation’s opioid crisis.

Johnson seems to think it's possible, or at least he did this summer. “The Medicaid expansion may be fueling the opioid epidemic in communities across the country,” Johnson wrote to the Health and Human Services inspector general in July. “Because opioids are so available and inexpensive through Medicaid, it appears that the program has created a perverse incentive for people to use opioids, sell them for large profits and stay hooked.”

The theory’s plausibility crumbles in the face of available evidence.

Begin with a logical assumption that has been broadly accepted since Aristotle walked the earth: For A to cause B, A has to occur before B in time. The Affordable Care Act was passed in 2010, but the national expansion of Medicaid didn’t begin until 2014. That’s after the 15-year-long quadrupling of prescription painkiller overdoses deaths from 4,030 to 16,234. Obviously those deaths can’t be attributed to a Medicaid expansion that hadn’t happened yet.

Opioid overdose mortality has continued to rise since 2014, but the primary driver of the recent growth has been the explosion of deaths from heroin, much of it laced with fentanyl (Chris Ingraham covered this in Wonkblog here). Medicaid can’t be blamed for those deaths because heroin is from the black market rather than from the health-care system to which insurance gives access. Indeed, because Medicaid is the lead payer for treatment of heroin addiction in much of the country, without it, heroin overdose deaths would almost surely be even higher.

Recent results from Oregon’s Medicaid Experiment also bear attention. The state did not have sufficient funds to cover everyone who wanted Medicaid, so they chose among applicants using a lottery, thereby creating a rigorous randomized study of Medicaid’s impact on low income people. In a comparison of 6,300 individuals who won the Medicaid lottery to 5,700 who didn’t, receiving Medicaid increased enrollees’ use of certain prescriptions (e.g., anti-depressants) but had no effect on their likelihood of being prescribed opioids.

Without doubt, at least some people on Medicaid receive opioids that they should not, either because they intentionally fake a pain condition or because their physician prescribes inappropriately. But this is also true of some enrollees in Medicare, Blue Cross and Aetna, among a hundred other insurers. All insurance programs, including Medicaid, should thus employ protective mechanisms and educational initiatives to make it more likely that opioids are only prescribed when appropriate.

But singling out the Medicaid expansion for a unique role in creating the opioid crisis is a credibility-straining exercise in bashing an essential part of the safety net.