Michael Sowers, a star of the Premier Lacrosse League, endured his fifth diagnosed concussion in 2021. His personal doctor told him he might want to consider retiring, but another physician had an idea that would keep him on the field.

Dr. Wayne Olan, a neurosurgeon at George Washington University Hospital in Washington, D.C., suggested Sowers wear a silicone collar around his neck made by a company he serves as a medical adviser. Called Q-Collar and costing $199, the device restricts the flow of blood from the head, and, if science touted by the company is accepted, gives the brain an extra layer of cushioning.

“I can’t think of anything we can do that is so simple but also so important,” Dr. Olan, who also coaches high school lacrosse, said in an interview.

But does the Q-Collar, whose origin story involves a novel analysis of the anatomy of a woodpecker, actually protect the brain? Football players on more than two dozen college and N.F.L. teams are wearing it as they search for something, anything, that can keep them safe. Still, serious doubts have emerged about the science behind the device, according to an extensive review of government documents and scientific studies by The New York Times, as well as interviews with scientists who have examined research into the Q-Collar.

Far from making athletes safer, some experts in brain injuries and neuroscience say, the Q-Collar may embolden them to take risks they otherwise wouldn’t.

“The danger with a device like this is that people will feel more protected and play differently and behave differently,” said James Smoliga, a professor of physiology at High Point University in North Carolina who has led a crusade in academic journals against the device.

Sowers, 25, the lacrosse star, seemed to validate this concern. “I can go out there and play my game,” he said. “I don’t have to fear the contact.”

Q30 Innovations, the company that developed the Q-Collar, and experts like Dr. Olan have stood by their claims that the device can make players who wear it safer, with rule changes and safety protocols mitigating any tendencies toward recklessness.

“We’re not talking about concussions,” Tom Hoey, the company’s chief executive, said in an interview. “We are talking about the repetitive hits,” he said, adding, “the Q-Collar reduces the injury and changes to the brain caused by subconcussive impacts.”

Q30 Innovations, based in Westport, Conn., had a significant triumph last year when the F.D.A. approved the Q-Collar for sale as a medical device in the United States.

The agency declared that company-funded studies had shown it might limit damage to brain tissue. In November, the International Bobsleigh and Skeleton Federation approved the Q-Collar for use in competition. Meghan Klingenberg, who plays for the Portland Thorns of the National Women’s Soccer League, wears it. So do football players at over a dozen college teams including Auburn and Alabama and 12 to 15 N.F.L. teams. Several high school teams have adopted the collar, too.

The device, a lightweight, cushioned collar that slips around the lower neck, is snug enough to constrict the slightest bit of blood flow but not too tight to cause discomfort.

Drue Tranquill, a linebacker for the Los Angeles Chargers, started wearing the Q-Collar this season. A hard hit on a punt play last year landed him in the NF.L.’s concussion observation program.

“I wanted to protect myself,” Tranquill said in a recent interview. In early October, though, the F.D.A. posted a summary of its decision that was far more measured than the February 2021 approval announcement. The summary, which the agency published to document its science, included a series of buyer beware caveats about the key study that led to its approval.

The agency cited uncertainty surrounding the imaging technology that the studies relied on. Those studies revealed, under certain circumstances, subtle changes in the brain tissue of high school football and soccer players who did not wear the Q-Collar, regardless of whether they experienced concussion symptoms. Those changes, detectable only with high-tech imaging, differed from changes in the brain tissue of the players who did wear it.

However, the F.D.A. stated, a link between the changes the studies revealed and actual brain injuries has not been “validated.” Also, the agency said, the scientists found something they did not initially say they were looking for, potentially making the results more uncertain.

“They’re finding stuff, but it feels like noise,” said Matt Tenan, a program director at West Virginia University’s Rockefeller Neuroscience Institute. He and other skeptics cite inconsistencies in the Q-Collar studies and do not accept the theory at the heart of the device — that compressing the jugular vein in the neck keeps additional blood in the cranium, allowing the blood to function like the white surrounding the yolk of an egg.

The company points to the 18 published studies supporting the concept while also acknowledging there is more research to be done.

At stake is a potential windfall from the more than $30 million and countless hours of research that have already been invested in trying to figure out the efficacy of the Q-Collar. Also, though, there is the health and safety of millions of athletes — pros and amateurs of all ages — and possibly soldiers who may eventually wear a device that may provide little more than a false sense of security.

A curious origin story and the biggest names in brains.

Dr. David Smith, an inventor and former practitioner of internal medicine, came up with the idea for the Q-Collar after discovering what he believed was the key to a woodpecker’s brain health — a neck muscle that contracts and traps blood in their brains when they peck, at trees, the ground, the siding on your house.

The theory ran counter to accepted bird research. Ornithologists have concluded that beaks and spongy skulls with unique musculature provide the cushioning, not jugular vein compression. Also, the brains of deceased woodpeckers have shown signs of brain damage.

And yet the longing for equipment that can prevent traumatic brain injury and make dangerous sports feel safe is intense.

The F.D.A. experts cited the urgent need for devices that “may” protect the brain from mild impacts in sports and the low risk of the Q-Collar.

“The probable benefits outweigh the probable risks,” the agency said.

The Q-Collar’s high-profile supporters include Dr. Julian Bailes, a NorthShore Medical Group neurosurgeon who was at the forefront of research into brain injuries in sports. Alec Baldwin portrayed Dr. Bailes in the movie “Concussion.”

Dr. Smith began his research by tossing small steel casings that held various amounts of blood from the roof of his office, but the initial small animal studies on jugular vein compression and one large animal study were performed in Dr. Bailes’s lab.

Early on, Dr. Bailes, who is a minority shareholder in Q30 Innovations, intuited that an extra teaspoon of blood in the brain might help keep the body’s most irreplaceable organ more still.

“If the brain doesn’t move, it doesn’t get injured,” Dr. Bailes said in an interview.

Scientists skeptical of the Q-Collar accept that premise. It’s the research behind the Q-Collar that they question.

Martha Shenton is a professor of psychiatry and radiology at Harvard’s Brigham and Women’s Hospital and an expert in the high-tech brain imaging that the Q30 scientists have relied on. At the request of The New York Times, she reviewed the findings of the key study the F.D.A. cited when it approved the Q-Collar.

Dr. Shenton raved about trying to protect the brain from inside the skull, but was less enthusiastic about the conclusions of the study.

“None of it makes sense,” Dr. Shenton said.

Gregory Myer, director of Emory University’s Sports Performance and Research Center, who has led the Q30-funded human clinical trials, allowed that much research remains to figure out the Q-Collar’s true potential.

“It is not a magic bullet,” said Dr. Myer, who in addition to being the primary researcher has also received money as a consultant to Q30 Innovations.

Dr. Myer said the evidence so far suggests the collar can be a piece of a safety puzzle that includes adjustments to rules.

“It all fits together into making sports safer,” he said.

If Dr. Myer is right, the payoff could be significant. Hoey, the Q30 chief executive, said the company expects $100 million in sales during the next five years from just 1 percent of its target market.

More than two million students in the United States played high school football, lacrosse, hockey or soccer last year. All are considered activities with a high risk of subconcussive head impact. Millions more play at the youth and college level.

The company has spent roughly $550,000 on lobbyists and consultants to help in its dealings with the Department of Defense.

In October, Q30 announced it had received a $2.8 million research contract from the U.S. Army to study whether the Q-Collar can help lower the risk of brain injury to soldiers exposed to explosions.

A pivotal study garners a key approval — and questions.

Dr. Smith first studied woodpecker anatomy some 15 years ago. He then began discussing Q-Collar prototypes with Dr. Joseph Fisher, a physiologist and expert in brain blood flow at the University of Toronto.

Along with Dr. Bailes, they spent three years building computer models and studying the effects of jugular compression on pigs and rodents, eventually growing convinced the Q-Collar could be a game changer. In 2012, Dr. Bailes contacted Hoey. Q30 Innovations had been making an innovative, flavored mouth guard, but after speaking with Dr. Bailes the company decided to go all in on the Q-Collar.

“From the very beginning we decided the data had to drive the safety,” Hoey said. “We reached out to the best thought leaders in North America.”

In 2015, Performance Sports Group, the company behind several well-known sporting goods brands, such as Bauer in hockey and Easton in baseball and softball, licensed the Q-Collar for sports uses for $7 million. It also invested $1 million in Q30 Innovations. It put on a presentation in Midtown Manhattan for potential investors, trotting out the hockey legend Mark Messier to attest to the Q-Collar’s potential. A year later it filed for bankruptcy.

Q30 Innovations regained full control of the device in 2018. Scientists knew by then the Q-Collar could not prevent concussions. Athletes including Luke Kuechly of the Carolina Panthers had had at least one concussion while wearing it. Dr. Myer said the company needed to find an objective measure that showed wearing the device could at least lower the risk of brain damage.

In 2018, Dr. Myer and his team began studying 284 high school football players on seven teams to see if scans of their brains before and after the season would reveal any differences between players who wore the Q-Collar and those who did not.

The players wore accelerometers inside their helmets to count the number of hits and their impact. When the scans were combined into a composite image for each group, they revealed, at certain levels of impact, microscopic changes in one area of the brain for some players who did not wear the collars and substantially different changes for those who wore them.

However, experts in the high tech imaging, known as D.T.I., cautioned against drawing too many conclusions based on those results.

Derek Jones, a professor at Cardiff University’s Brain Research Imaging Centre, in Wales, described D.T.I. technology as “very sensitive but not very specific.” He said the data it produces is difficult to interpret, especially in the complex regions of the brain that had produced the most significant results in the Q-Collar studies.

Dr. Shenton, the Harvard specialist, questioned the Q30 scientists’ interpretation of the data from their studies. She said the numbers reported go in the opposite direction of what a brain scientist might predict, and only in one narrow range of severity.

“They say, ‘We get a change and it doesn’t matter the direction,’” Dr. Shenton said. “It’s so not what you would expect.”

Tom Talavage, head of the biomedical engineering department at the University of Cincinnati, who served as the brain imaging expert for the study, said damage to brain tissue from subconcussive hits like those that athletes in contact sports experience can differ from other forms of brain damage. He agreed, though, that interpreting data from this complex area of the brain can be fraught.

Dr. Myer, the primary investigator on the studies, acknowledged researchers can’t yet draw any concrete conclusions.

“Hard to explain exactly what the results mean but certainly a target for future research,” he wrote in a recent email.

Dr. Fisher, the co-inventor, said the lack of slam-dunk proof has allowed critics to “moan and groan,” as he put it, but no one in his family rides a bike or skis without wearing a collar. “Let’s say it does nothing, then you lose nothing,” he said.