Anandamide Climbed Through a Marathon, but Only 3 of 19 Runners Said They Felt a Runner's High

Twenty-eight kilometers into a marathon, two laps of Lake Baldeney in Essen, Germany, behind them, 19 trained runners had a needle in the arm and a rating slider in hand. Asked how euphoric they felt, they reported about the same as they had when they walked the identical route weeks earlier. Their blood told a different story.

That small mismatch sits at the center of a paper published September 1, 2026 in BMC Medicine. Researchers led by Michael Siebers, a psychiatrist at LVR University Hospital Essen, with senior author Johannes Fuss of the University of Duisburg-Essen, tracked a chemical long suspected of powering the runner's high. The chemical behaved as predicted. The feeling was another story.

What the blood draws showed

The older textbook story credits endorphins. The newer one points to endocannabinoids, small fat-soluble molecules that cross into the brain and bind the same receptors that THC targets. Anandamide and a second compound, 2-AG, are the best studied. Fuss's group reported in a 2015 mouse experiment that wheel-running animals relied on cannabinoid receptors, not opioid receptors, for their drop in anxiety and pain sensitivity, according to PsyPost's coverage.

Human data had a gap. A 2022 systematic review from the same group found that 14 of 17 studies detected an endocannabinoid rise after a single bout of exercise, but nearly all of those workouts lasted under an hour. Nobody had much data from people running for four, ten or thirty hours.

The new paper has two parts. In Study 1, 19 trained runners (7 women, 12 men) ran the marathon, giving blood at the start, at 14 and 28 kilometers, at the finish and after 45 minutes of rest. One to two months later each returned to walk the same route for the same length of time. In Study 2, 36 athletes at the 2024 TorTour de Ruhr ran 100, 160 or 230 kilometers and gave blood before and after.

Anandamide climbed through the whole marathon and was still elevated at the 45-minute mark. Walking produced only modest changes. The runners also worked harder: about 83 percent of estimated maximum heart rate on the run versus about 58 percent on the walk. After every ultramarathon distance, both anandamide and 2-AG sat above pre-race levels.

The chemistry and the feeling do not line up cleanly

Marathoners did report more euphoria and less anxiety than walkers at several points. By the 28-kilometer mark, though, euphoria matched the walkers, and pain ratings rose sharply from there. In the ultramarathon group, anxiety dropped after the race, pain went up and euphoria did not change in a statistically significant way. Of the ultramarathoners asked afterward, 12 said they had a runner's high and 19 said they had not.

Siebers described the arc runners know from the inside. "At the beginning of a run, everything may feel uncomfortable," he told PsyPost. "Your calves are aching, your back feels stiff, and every step requires some effort." Later, he said, movement smooths out and some people reach a state of flow.

“It's important to recognize that this study doesn't establish that increases in AEA are responsible for the 'runner's high.'”

Fuss said the same in the university's press release, as reported by News-Medical: the team cannot yet conclude that the signaling molecules are responsible for the runner's high. The study also did not check whether the runners with the biggest anandamide rise felt the most calm or euphoria.

Why the sample matters

The limitations are listed in the paper itself. Sample sizes are small. The marathon and ultramarathon designs differ, so the two cannot be compared directly. Walking is a lower-intensity condition rather than a neutral baseline, and it was tested weeks apart from the race. Euphoria and anxiety were each rated on a single-item scale, which is a blunt tool for complex feelings. A post-race comparison of anandamide between the marathon and ultramarathon groups was exploratory, and the authors suggest the shorter, faster race may explain the higher marathon values.

What the data does support is narrower and still interesting. The endocannabinoid system stays switched on for hours of running, well past the one-hour window most earlier work covered. Whether that translates into the glow some runners describe, or simply tracks effort, is the open question. Only a study that links each runner's blood level to their own rating can answer it.