Story highlights One in 20 adults -- a quarter of a billion people between 15 and 64 -- used at least one drug in 2014

Heroin use in the United States is up 145% since 2007, the report found

(CNN) Worldwide, drug use has remained steady over the past four years, according to a report released today by the United Nations Office on Drugs and Crime. However, researchers found that heroin use in the United States is up 145% since 2007.

One in 20 adults -- roughly a quarter of a billion people between ages 15 and 64 -- used at least one illegal or improperly used drug in 2014, according to the World Drug Report 2016. Though the numbers have not grown in proportion to the global population, new trends have developed, including increased sales in anonymous online marketplaces.

"The darknet is becoming a very popular way for people to get drugs," said Joseph J. Palamar, an assistant professor at NYU Langone Medical Center who was not involved in the new research.

"This raises concerns in terms of the potential of the 'dark net' to attract new populations of users by facilitating access to drugs in both developed and developing countries," the authors wrote.

The darknet is not the only marketplace for illicit drugs. Palamar said new psychoactive compounds that are chemically similar to illicit drugs are legally sold and purchased on the "regular" Internet. He described some sites as being "pretty much like eBay," complete with user ratings that drive business to those selling higher-quality, more pure drugs.

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