She is well aware that many in the official yoga industry would disapprove of her 420 class and doesn’t care. “I find it to be a valuable tool in teaching,” she said. “Disbelief is my biggest obstacle. People don’t believe that they can feel their heart beat or that they can send breath into their lower appendages. A little pot relaxes them into comprehending. And if you want to just lie down in my class, that’s O.K., too.”

William Sands, dean of the College of Maharishi Vedic Science at Maharishi University of Management in Fairfield, Iowa, represents the stringent opposition. He has just finished writing a book on Maharishi Mahesh Yogi, who mentored the Beatles and introduced the West to transcendental meditation. Dr. Sands said that “marijuana inhibits the ability to experience yoga — the inner self — and is therefore incompatible with the practice of transcendental meditation.” To Dr. Sands, yoga is the full package: a physical and mental discipline.

Others make a distinction between the kind of discipline required for yoga and for meditation.

Ms. McDonald said she understands why certain yogis disapprove of pairing the practice with pot. “Some devotees have literally dedicated their lives to yoga,” she said, “and as a result they tend to see it as an ideal, beyond human flaws, capable of our salvation. I think that perfection is found by authentically being in the present moment, rather than as an outcome of enlightenment. So I’m not afraid to admit I’m not perfect and come out of the closet on these issues.”

Dee Dussault of San Francisco is another rare instructor who advertises what she teaches, in her case Ganja Yoga. Ms. Dussault traces the division between what she calls tantric yoga and straight yoga to the Buddha, who advocated purity. “Tantric yoga says you use whatever tools are available to get to a place of transcendence,” she said, adding the caveat that marijuana must be used “mindfully.”

Yoga and herb intake have been linked since ancient times. The yoga sutras, written in Sanskrit before the time of Christ, are considered the practice’s foundational text. The sutras list herbs as one of five methods to lift the veil of ignorance, or the barrier between the conscious and the unconscious. Dr. Sands, though, expressed skepticism, saying that the “herbs” in question could be as uncontroversial as cardamom. “People who interpret the sutras’ use of ‘herbs’ as marijuana,” he said, “are looking for a rationale.”

But Mark Haskell Smith, the novelist and author of the nonfiction “Heart of Dankness: Underground Botanists, Outlaw Farmers, and the Race for the Cannabis Cup ,” is pretty sure that the sutras do not refer to cardamom. A yoga practitioner for 20 years, Mr. Smith said he occasionally uses pot when he practices. When he does, he said, he finds that “I go more deeply into the asanas,” or poses.

“Part of the point of yoga is to relax the body,” he said. “And marijuana helps a lot of people to do that.”