NOVATO, Calif. — A former pro football star and an advocate for athlete marijuana use have teamed up to open a gym in San Francisco that they say will be one of the first in the world to allow members to smoke pot while working out.

Former running back Ricky Williams, who played for the Saints, Dolphins and Ravens, and Jim McAlpine, a snowboard company executive, said Power Plant Fitness also will offer edibles and topical gels for those who don’t like smoking the plant. They say using pot while exercising can help them focus or relax.

Members of the gym, which plans to open this year, will need a medical marijuana prescription to join, but that could change if California voters legalize recreational pot in November.

“I personally use it for focus. It’s not about getting high. It’s about keeping my mind engaged in the activity I’m in,” said McAlpine, who organizes the 420 games, athletic events aiming to stop the stigma against pot use.

Carla Lowe, founder of Citizens Against Legalizing Marijuana, a political action committee based in Sacramento, said it’s not clear how the drug affects the body but “there’s zero evidence that marijuana helps you focus. There is evidence that it makes you dopey.”

Her group is working to defeat the ballot measure that would legalize marijuana in California, saying it “does not bode well for the future of our country.”

But Williams, who was suspended several times by the NFL for marijuana use, said he wants to dispel the stigma.

“I think a lot of people buy into the stoner stereotype where guys just sit on the couch, smoke and don’t do anything, and they’re not very motivated,” said Williams, who retired from the NFL after the 2011 season. “I found when I was playing football that using cannabis helped me relax physically, relax mentally and even spiritually.”

Any potential benefits of marijuana on exercise have not been studied thoroughly. But one doctor who works with marijuana-smoking patients says the drug can help manage post-workout pain.

“To use cannabis in that sense for pain relief instead of the usual things you’re able to use now, like opioids, is hands down why you would use it,” said Dr. Perry Solomon, chief medical officer for HelloMD, a digital health care platform for the cannabis industry.