As voters and lawmakers in more states decide to legalize marijuana, policy makers will have to answer a fresh and difficult question: How should governments regulate the production and sale of the drug?

Beyond keeping marijuana out of the hands of minors, a good regulatory system has to limit the increase in drug abuse that is likely to accompany lower prices and greater availability after legalization. It should protect consumers from both dangerous and counterfeit products, reducing the physical risk from a psychoactive substance. And a well-regulated system should undermine and eventually eliminate the black market for marijuana, which has done great damage to society.

The experiences of Colorado and Washington, where sales of recreational marijuana started this year, will prove instructive. While there are important differences in their approaches, both states have licensed businesses to grow, process and sell marijuana while imposing strict rules and high taxes on them. Other states that legalize will probably adopt a similar model, because it resembles how the federal and state governments regulate tobacco and alcohol.

A Better Way to Tax Policy makers trying to regulate the drug will face challenges similar to the ones American lawmakers faced at the end of Prohibition in 1933. Like alcohol during Prohibition, marijuana is widely available across the United States today. But it will become much more accessible after legalization, when businesses engaged in its production and sale no longer operate in the black market nor engage in violence. The pretax price of the drug could fall by 90 percent after legalization, according to Robert MacCoun, a law professor at Stanford, and fellow researchers. Moreover, marijuana businesses will have a financial incentive to get a broad population to use the drug regularly. A recent report prepared for the Colorado Department of Revenue concluded that nearly 90 percent of the demand for marijuana in the state this year would come from only 30 percent of users, those who use the drug 21 to 31 days a month.