Studies on the effects of marijuana on athletes are sparse. “Most of the work is, at the moment, observational, looking at people who use and don’t use and comparing them,” said Dr. Bonn-Miller, who is conducting studies on the use of cannabinoids among former professional football players. “There hasn’t been a whole lot of funding for this.”

A 2017 survey in the Journal of Science and Medicine in Sport found only 15 published studies that investigated the effects of cannabis and its main psychoactive ingredient, THC, on exercise performance. “It is generally considered that THC won’t improve aerobic performance and strength, and my review confirms that impression,” said Dr. Michael C. Kennedy, a cardiologist, clinical pharmacologist and associate professor at the University of New South Wales and St. Vincent’s Hospital Medical School, who conducted the review.

He doubts claims that it helps with recovery and improves concentration, and says that athletes who tout its athletic virtues are just promoting cannabis use. “It will not make you faster, it may slow you down and certainly should not be used if there is any possibility of heart disease,” Dr. Kennedy said. Indeed, some studies have linked marijuana to hypertension and other heart risks.

But Dr. Bonn-Miller believes that from a physiological standpoint, the relationship between marijuana use and running makes some sense. “There’s a lot of overlap in terms of the pathways that are activated between what’s known as a runner’s high and the high that comes from THC,” he said. “Both of those involve activation of the endocannabinoid system, so it’s not too surprising that THC might be used to enhance the runner’s high that’s gained from endurance exercise.”

Runners also report using products with cannabidiol, or CBD, a nonpsychoactive component of marijuana that has shown to have anti-inflammatory properties, for recovery. The CBD is usually applied through an oil. “It lowers the amount of many, many pro-inflammatory cytokines — things that our body makes naturally in response to any inflammation response,” said Dr. Orrin Devinsky, director of the Comprehensive Epilepsy Center at NYU Langone Health, who is studying the use of CBD to treat epilepsy. He said that CBD has also been shown to bind to serotonin receptors, “which may be related to its effect as an anti-anxiety agent.”

Scott Dunlap, a 48-year-old ultrarunner who calls himself a semiprofessional (he has sponsors but also a day job) and who once ran a race in a marijuana leaf costume, says he will use an edible or vape marijuana after a long race. (He tried it once during a run and said he wound up “lost and hungry.”) He doesn’t see using marijuana after running a race as all that different from drinking a beer — except that a lot of races provide beer free.

The 420 Games, which has events in California, Colorado and Pennsylvania, gives out samples — sometimes marijuana, but more often oils and creams containing CBD — in places where they are legal, though the sponsors say they are not intended to be used at the event.

“I can honestly say it’s one of my favorite events of all time,” said Mr. Collins, the ultrarunner, who previously served as a spokesman for the event.