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Want to get stoned without breaking federal law? Try exercise. Past research suggests it may trigger the release of natural marijuana-like substances in the brain, and now a small but intriguing new study finds that exercise reduced marijuana use and cravings in heavy pot smokers who weren’t even trying to quit.

The new study, which was published in PloS One, involved only 12 volunteers, all of whom were heavy daily marijuana smokers. Over the course of two weeks, the participants exercised on a treadmill 10 times, each time for 30 minutes at 60% to 70% of their maximum heart rate. (More on Time.com: Want to Improve Your Memory? Try Taking a Walk)

Before and after each session, they were exposed to marijuana-related images on a computer screen, such as pictures of bongs, weed and people getting high. Then they rated their craving for a toke. By the end of the first week, cravings were down 50%. The participants went from smoking, on average, 5.9 joints to 2.8 joints a day.

Since the study wasn’t randomized, it’s impossible to tell whether the exercise itself caused the reduced marijuana use and craving, or whether it was simply a byproduct of being involved in an experiment focused on addictive processes. (More on Time.com: Are Stoners Dumb or Do They Just Think They Are?)

Still, there’s a plausible biological reason to think that exercise can cause a “high” similar to that from smoking pot. For starters, a 2003 study in college students found that using a treadmill or stationery bike at 60% to 70% of max heart rate for 50 minutes significantly raised levels of anandamide, a cannabinoid that occurs naturally in the brain and body. THC, a psychoactive ingredient in marijuana, activates the same brain receptor that anandamide does.

Other studies show that mice bred to lack cannabinoid receptors in the brain run only half as much as normal mice do when given the opportunity — suggesting that they find it less enjoyable. Research also finds that rats given a drug that blocks cannabinoid receptors don’t experience the normal brain-cell growth that typically accompanies running. (More on Time.com: How to Make Marijuana Legalization Smart)

But this doesn’t mean that the substances traditionally said to cause runner’s high—the body’s heroin-like compounds, endorphins—aren’t also part of the experience. Increases in levels of endorphins and other brain opioids are seen during exercise.

And if you block the receptors for these natural opioids with naloxone (a drug that also reverses overdose if given in time) or a longer acting drug called naltrexone, studies find that you can prevent people from feeling the lift in mood and reduction of anxiety that typically follows exercise. Other research has found that people exercise less intensely and perceive that the exercise is harder if given opioid blockers.

Oddly, however, a recent study found that blocking opioids with naltrexone in heavy marijuana smokers actually increased their high from smoking pot. (More on Time.com: Should Overdose Antidote Be Made More Available?)

Given that the brain’s cannabinoid and opioid systems clearly interact, it’s not surprising that they would both be involved in the exercise-induced runner’s high. And so, for marijuana smokers, exercise could be a real gateway drug: to “harder” (in both senses of the word) but natural highs!