Medical marijuana a challenge for legal pot states: how to get customers into taxed system?

A year into the nation's experiment with legal, taxed marijuana sales, Washington and Colorado find themselves wrestling not with the federal interference many feared, but with competition from medical marijuana or even outright black market sales.

In Washington, the black market has exploded since voters legalized marijuana in 2012, with legally dubious medical dispensaries opening and some pot delivery services brazenly advertising that they sell outside the legal system.

In Colorado, the number of patients on the medical marijuana registry went up, not down, since 2012, meaning more marijuana users there can avoid paying the higher taxes that recreational pot carries.

Officials in both states say they must do more to drive customers into the recreational stores. They're looking at fixing the big tax differential between medical and recreational weed.