Gary Morrison / Getty Images

Working out could give pot users an extra high — but that may not be as much a boon as users might think.

The study, published in Drug and Alcohol Dependence, found that levels of THC—marijuana’s active ingredient— were higher by an average of 15% immediately after users exercised. And that could be enough to trigger a positive blood test and suggest that users got high more recently than they actually did.

Although it’s been known for years that THC is stored in fat and can sometimes cause positive tests weeks after a person has last smoked it, the study is the first to document that exercise can lead to a rise in its levels in the blood. The data may explain cases like that of a jockey who had to lose weight for a race and tested positive for cannabis long after he’d said he had last smoked it— and instances of users testing positive in rehab when they exercise and lose weight despite being abstinent.

The study involved 15 regular marijuana users who smoked an average of a joint a day and submitted to a blood test both before and after a 35 minute bout of moderate exercise on a stationary bike. They had been abstinent for at least 24 hours when they were tested.

The researchers also measured their body fat, by recording their body mass index (BMI) and found that changes in THC levels after exercise, which ranged from zero percent to 34%, increased with body fat. “People with larger BMI showed a bigger increase [and] very thin people [had] no effect,” says the study’s lead author Iain McGregor, professor of psychopharmacology at the University of Sydney in Australia. The rise was short-lived, lasting for less than two hours.

Was it enough to cause impairment? That’s not clear because the study wasn’t designed to measure intoxication. McGregor says, however, that it’s “theoretically” possible, and is conducting more research to answer that question.

MORE: Marijuana: The Next Diabetes Drug?

Other research found that even in people who do not smoke marijuana, both exercise and stress can increase activity of the brain’s natural cannabinoid system and lead to the release of the body’s THC equivalents. Those agents contribute to “runner’s high” and other pleasant, dreamy feelings during workouts or stressful experiences. McGregor is hoping to investigate how the exercise-induced release of THC compares to the effects of the natural cannabinoid system, which is important in regulating stress and pain.

In the meantime, understanding how a “second hit” of THC from exercise—and traumatic events— could affect judgment and behavior will be critical to figuring out the role that THC testing can play in a variety of settings, from roadside stops of impaired drivers to autopsy reports to determine cause of death.

While the amount of THC detected in the blood suggests that it’s unlikely that the second hit will cause significant impairment, if additional studies suggest that it does, scientists and regulators may face some thorny questions with respect to stoned driving. If blood levels of THC are high after a crash, for example, would researchers be able to determine if the driver were stoned when he got behind the wheel, or experienced a surge of THC from prior use that was released by the stress of accident? Would the scientists be able to document whether the impairment were similar or qualitatively different in each circumstance? That would determine, in turn, whether teasing apart where the THC originated — from recent use or from stored fat — would matter for law enforcement purposes. Given the interest that this issue will generate, McGregor hopes these questions won’t be unanswered for long.