Listen to a podcast of NPR's David Kestenbaum interviewing yours truly for Planet Money.  Here's the summary:

Fourteen states have adopted medical marijuana laws. We talk to Harvard economist, Jeffrey Miron, about what happens when drugs move from the black market to the open market. Do they get 100 times cheaper? Or instead, more expensive? Miron talks about the economics of prohibition, and reveals his drug of choice (which is legal) and one he would like to try (which is not).