It was a shopping spree to end all shopping sprees. Armed with a physician's letter that allowed them to buy medical marijuana and $400, the volunteers were to go from dispensary to dispensary to buy edible cannabis products ranging from baked goods and beverages to candy.

Then the researchers got to work. They crushed or mixed the entire package of contents and tested the levels of THC and CBD -- the active ingredient in marijuana -- to see how they matched up with the labels on the packaging.

Few of the products passed the test.

Of the 75 products made by 47 different brands that were investigated, only 17 percent were accurately labeled.

Twenty-three percent were underlabeled (meaning they were more potent than advertised) and 60 percent were overlabeled (meaning they had less of the medicinal ingredient than they promised).

Of the three cities involved in the study -- San Francisco, Los Angeles, and Seattle -- products in Los Angeles were more likely to be overlabeled and those from Seattle underlabeled.

"Edible cannabis products from 3 major metropolitian areas, though unregulated, failed to meet basic label accuracy standards for pharmaceuticals," the researchers, led by Ryan Vandrey, an associate professor at Johns Hopkins University School of Medicine, wrote.

In fact, some of the products contained negligible amounts of THC and "may not produce the desired medical effect," they warned. Others contained significantly more "placing patients at risk of experiencing adverse effects."

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