These days, Americans shop for nearly everything online -- including marijuana. That's the conclusion of a new study published today in the American Journal of Preventive Medicine led by San Diego State University Graduate School of Public Health associate research professor John W. Ayers. Millions are searching for and finding online marijuana retailers across the country, the researchers find.

The team monitored Google searches in the United States between January 2005 and June 2017, including all searches with the terms marijuana, weed, pot, or cannabis combined with the terms buy, shop, or order (for example., "buy marijuana"). They omitted similar but irrelevant searches like "buy weed killer." The team then replicated the relevant searches and checked to see whether the resulting websites advertised mail-order marijuana.

"By studying anonymized, aggregate Internet searches and search results, we were able to directly observe the online marijuana marketplace," said study coauthor Mark Dredze, the John C. Malone Associate Professor of Computer Science at Johns Hopkins University.

The team found marijuana shopping searches nearly tripled in the United States from 2005 to 2017, peaking between 1.4 and 2.4 million searches each month.

Marijuana shopping searches were highest in Washington, Oregon, Colorado, and Nevada. However, the annual growth rate in searching for these terms increased in all but two states, Alabama and Mississippi, suggesting demand is accelerating across the nation. (The six least populated states were excluded from the study.)

Forty-one percent of all search results linked to retailers advertising mail-order marijuana, promising delivery using a variety of methods including the United State Postal Service, commercial parcel companies such as UPS, or private courier. Moreover, mail-order marijuana retailers occupied half of the first-page results, and three out of every four searches resulted in a mail-order marijuana retailer as the very first suggested link.

"Anyone, including teenagers, can search for and buy marijuana from their smartphone regardless of what state they live in," Ayers said.

Such online sales of marijuana are prohibited in the United States, even in states that have legalized or partially legalized the drug, "but clearly these regulations are failing," said coauthor Eric Leas, a research fellow at Stanford University.

Public health leaders must immediately take action to curtail online marijuana sales, urged Theodore Caputi, the study's lead author and George J. Mitchell Scholar at University College Cork.

"Children could obtain marijuana online without safeguards to protect them," he said. "States that have legalized marijuana might not be able to collect taxes to offset the public health costs of legal marijuana from online retailers, and the instant online availability of marijuana could increase marijuana dependence among all age groups."

One solution could be for public safety officials to work with internet service providers to purge marijuana retailers from major search engine results, said Ayers. Such a move would "effectively close off illicit retailers from consumers," he said.