Studying illegal drug use nationally from 2000 to 2010, researchers found the amount of marijuana consumed by Americans increased by more than 30 percent from 2006 to 2010, while cocaine consumption fell by about half. Meanwhile, heroin use was fairly stable throughout the decade.

Methamphetamine consumption dramatically increased during the first half of the decade and then declined, but researchers did not have enough information to make a credible estimate of the drug's use from 2008 to 2010.

The findings come from a report compiled for the White House Office of National Drug Control Policy by researchers affiliated with the RAND Drug Policy Research Center.

"Having credible estimates of the number of heavy drug users and how much they spend is critical for evaluating policies, making decisions about treatment funding and understanding the drug revenues going to criminal organizations," said Beau Kilmer, the study's lead author and co-director of the RAND Drug Policy Research Center. "This work synthesizes information from many sources to present the best estimates to date for illicit drug consumption and spending in the United States."

Because the project only generated estimates through 2010, researchers say the report does not address the recent reported spike in heroin use or the consequences of marijuana legalization in Colorado and Washington. The report also does not try to explain the causes behind changes in drug use or evaluate the effectiveness of drug control strategies.

The study, published on the website of the Office of National Drug Control Policy, provides estimates of the amount of cocaine, heroin, marijuana and methamphetamine used each year from 2000 to 2010. The study includes estimates of retail spending on illicit drugs and the number of chronic users, who account for a majority of drug consumption.

Researchers say that drug users in the United States spent on the order of $100 billion annually on cocaine, heroin, marijuana and methamphetamine throughout the decade. While the amount remained stable from 2000 to 2010, the spending shifted. While much more was spent on cocaine than on marijuana in 2000, the opposite was true by 2010.

"Our analysis shows that Americans likely spent more than one trillion dollars on cocaine, heroin, marijuana and methamphetamine between 2000 and 2010," Kilmer said.

The surge in marijuana use appears to be related to an increase in the number of people who reported using the drug on a daily or near-daily basis.

The estimates for marijuana are rooted in the National Survey on Drug Use and Health, which surveys nearly 70,000 individuals each year. Estimates for cocaine, heroin and methamphetamine are largely based on information from the Arrestee Drug Abuse Monitoring Program, or ADAM. The final estimates also incorporated information from other data sources

However, since the federal government recently halted funding for ADAM, researchers say it will be considerably harder to track the abuse of cocaine, heroin, and methamphetamine in the future.

"The ADAM program provided unique insights about those who abused hard drugs and how much they spent on these substances," said Jonathan Caulkins, a study co-author and the Stever Professor of Operations Research and Public Policy at Carnegie Mellon University. "It's a tragedy that 2013 was the last year for ADAM. It is such an important data system for understanding drug problems."

To improve future estimates, the report recommends investments in programs like ADAM that collect detailed data from heavy users. It also recommends that federal agencies revise some of the questions on existing self-report surveys.

The study, "What America's Users Spend on Illegal Drugs, 2000-2010," can be found at http://www.whitehouse.gov/ondcp. Other authors of the report are Susan Everingham, Greg Midgette, Rosalie Pacula, Rachel Burns, Bing Han and Russell Lundberg, all of RAND, and Peter Reuter of the University of Maryland.

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