How much do Americans love weed?

According to Jonathan Caulkins, a professor of public policy at Carnegie Mellon University, Americans spend about 15 billion hours a year high on marijuana. He doesn't believe this is a good number to have arrived at.

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Caulkins, author of "Marijuana Legalization: What Everyone Needs To Know," which will soon be out in a new edition, derived the figure by analyzing the 2013 National Survey on Drug Use and Health, which surveys 70,000 individuals.

  • According to Caulkins’ analysis of the survey, there were an equivalent of 282 million days of use in the last 30 days reported by respondents. Under-reporting increases this by up to 33 percent.
  • He then multiplied that by number of sessions per use day, which vary by user types but which tend to be skewed because most  "come from daily and near-daily users who have multiple use-sessions per days of use,"
  • Finally, sessions are multiplied by hours of intoxication which can range from 1 to 4.

The habitual users are the real story, Caulkins says.

"People have not noticed that while we've been liberalizing drug policy, the country has developed a serious [problem] with people whose marijuana use is dominating their lives."

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A 2013 Pew poll showed that approximately half of the country has now tried marijuana, compared with 38 percent in 2003. Fifty seven percent of "millennials" had used it as of 2013, according to the survey. Regular users have increased to 7.5 percent from 6.2 percent in 2003.

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Caulkins says this raises questions about the cost of legalization. In his estimation, the savings to the criminal justice system, as well as the societal increase of marginally happier people, could be overwhelmed by the increased costs to the health care system of treating habitual users whose numbers are now increasing. This latter group also tends to have lower incomes: College graduates, he says, account for only 7 percent of all marijuana purchases.

"Even if the hours by people who are not dependent are a win, the hours by people who are dependent are a loss, and a bigger loss," he said.

Rob covers business, economics and the environment for Fusion. He previously worked at Business Insider. He grew up in Chicago.