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The term "runner's high" describes the feelings of well-being often associated with long workouts—not just running, but anything lengthy with repetitive motions like cycling and swimming. The phenomenon has been linked with helping runners get through slumps during marathons or other athletic challenges. In fact, people who exercise regularly have lower levels of depression. It may be part of the reason that the sport of running has become more popular around the world.

Runner's highs have long been linked to a group of hormones called endorphins, but while endorphins provide pain relief in the body, they are unable to communicate directly with the brain, so it is unlikely they create these mood-altering effects. Instead, we can credit another group of chemicals—endocannabinoids—which can pass through the brain's membrane. Endocannabinoids reduce anxiety and pain sensitivity, similar to the effects of THC in marijuana.

The endocannabinoid system is incredibly important, but the research surrounding it only stems back a couple of decades. In the early 1990s, the discovery of the cannabinoid receptors anandamide (AEA) and 2-arachidonoylglycerol (2-AG) uncovered the complex web of "the body's own cannabinoid system." Now, the research is trying to determine its functions and how it interacts with other substances, including medical marijuana.

Along with runner's highs, endocannabinoids have been associated with other positive effects on health such as protecting against depression and managing appetite and metabolism. They have also been linked to lower levels of inflammation and digestive distress, fewer migraines, and may even protect against seizures in epileptics. Although there is still much to be learned about this lesser-known physiological system, every indication suggests that everyone could use a little more of this kind of "cannabis" in their life, even if they hate running.