Smaller-dose pot-infused cookies, called the Rookie Cookie, sit on the packaging table at The Growing Kitchen in Boulder, Colo., in 2014. (Brennan Linsley/AP)

CORRECTION: An earlier version of the cited study incorrectly described how edible potency varies by city. Edibles in Los Angeles were more likely to contain more THC than promised, while those in Seattle were more likely to contain less.

Walk down any convenience store drug aisle and you can generally rely on the products on the shelves to contain the dosages they promise. But walk into a marijuana dispensary and you might not feel the same level of assurance.

That’s because the potency of marijuana edibles can be highly variable and have little relation to what is promised on the label, according to a new study published in the Journal of the American Medical Association.

The accuracy of 75 edibles from dispensaries in California and Washington, based on data published in the Journal of the American Medical Association. (Niraj Chokshi) The accuracy of 75 edibles from dispensaries in California and Washington, based on data published in the Journal of the American Medical Association. (Niraj Chokshi)

The results can’t be generalized due to a small sample size, but a review of dozens of products from marijuana dispensaries in California and Washington — two of the nation’s largest medical marijuana markets — found that 23 percent contained more active chemicals than their labels suggested, while 60 percent fell short on what was promised.

“There were some cases that were so extreme that they’re not just off by a little bit,” says study co-author Ryan Vandrey of the Johns Hopkins University School of Medicine. “We had a number of products that contained less than 25 percent of what was on the label.”

Vandrey and his co-authors tested 75 products from 47 different brands. They were considered accurately labeled if the levels of THC and CBD — two chemicals key to the desired effects of marijuana — were within 10 percent of the labeled values.

The edibles were purchased from dispensaries in Los Angeles, San Francisco and Seattle, with those from Seattle most likely to fall short on promised levels of the drug and those in Los Angeles most likely to contain more than specified.

Vandrey’s findings are probably not isolated. Tests commissioned by newspapers in Oregon and Colorado — among the first states to legalize marijuana for recreational use — showed that THC levels in edibles were all over the map as well.

“The state assures consumers that medical cannabis and cannabis-infused products undergo a battery of lab tests for everything from pesticides to potency before landing on dispensary shelves,” the Oregonian/OregonLive’s Noelle Crombie wrote this year, reporting the results of a three-month investigation. “Yet when it comes to potency, that promise is largely an empty one.” The Denver Post found similar variation last year.

Part of the problem, Vandrey says, is that the federal government typically regulates labeling.

“Because there’s a difference in federal and state law on marijuana, they can’t do that here and, in my mind, there should be more responsibility on the state level to then set something up,” he says.

And while it can be difficult to get dosages right — ensuring, for example, that all the brownies in a batch contain the same amount of active chemicals — it’s not impossible.

“The problem is 100 percent on the manufacturing,” Vandrey argues, adding that better regulation and enforcement would nudge producers to improve their processes.

Some states are moving to fix the problem, though. Washington is combining its medical and recreational marijuana rules, a process that Vandrey says may yield better regulation of edibles.