Recently Megan McArdle discussed whether legalizing pot would reduce hard drug use. Under marijuana prohibition, the buyer must enter an illegal drug market. The seller is likely to offer harder drugs for sale. By legalizing marijuana but keeping hard drugs illegal, the two markets become separated. Maybe marijuana users would be less likely to buy the harder stuff.

McArdle is not convinced:

I’m not sure how true this is — most people I know who tried harder drugs got them from the friends they smoked pot with, not the guy who sold it to them. On the other hand, it’s at least plausible ….

The only way to find out is to legalize and see what happens. Fortunately for the curious, two states are experimenting right now. We should have our answer in a few years.

While solid proof will take awhile, medical-marijuana laws may have provided some evidence already. Many states have laws that allow certain medical patients to purchase marijuana legally (currently 20 states and D.C. but more medical-marijuana states are probably on the way). These laws aren’t ostensibly meant to allow people to obtain marijuana for recreational use, but it is generally believed that this happens anyway. Initial results from these states indicate that easier access to marijuana might decrease heroin use. Luke Chu’s working paper finds that medical-marijuana laws lead to a decrease in arrests for cocaine and heroin possession (from 0 to 20 percent) and a 20 percent decrease in the number of people being treated for heroin (with no significant change in treatment rates for cocaine). The paper just received a revise and resubmit from the Journal of Law and Economics.