Doing so repeatedly is potentially fatal or life-shortening. Yet this is destined to become more than a headache, rather a potential multi-billion pound legal minefield which could change the face of contact sport, reducing participation worldwide.
Tottenham Hotspur were accused last month by Headway, the brain injury charity, of an "irresponsible and cavalier attitude" to concussion when they allowed French international goalkeeper Hugo Lloris to continue after having been knocked unconscious.
Headway say a player treated on the pitch cannot be accurately assessed in a few minutes. Romelo Lukaku, who accidentally clashed with Lloris, had himself suffered a month earlier after a head-clash in scoring for Everton - a goal he later said he could not remember. Also this season, Stoke defender Robert Huth has suffered concussion, and played on.
It's no easier to diagnose concussion off field, apparently. During the third Lions Test last summer, Australian flanker George Smith left the field most unsteadily. But recently-initiated International Rugby Board rules (pitch side concussion-assessment) allowed him back within minutes. Many rugby medics were horrified, but the IRB defended it.
Formerly, concussed players were mandatorily excluded for 15 days. Changing this rule has proved so controversial that former Ireland player Dr Barry O'Driscoll, after 15 years as an IRB medical advisor, resigned in protest. His public condemnation was damning: "They claim their five-minute assessment is based on best-research evidence, and best world practice. Not true. There is no evidence for a five-minute assessment. No evidence for world best practice because no other sport does it. It's an experiment no other sport will entertain . . . But this isn't a rugby experiment, it's a clinical medical experiment on a rugby field."
The inquest in August on the death of a 14-year-old Carrickfergus schoolboy, two days after he had been knocked out but allowed to play on, sent shock-waves throughout the rugby community.
Another seismic moment occurred at an IRB conference in Dublin last month, when Dr Willie Stewart, a neuropathologist at Glasgow's Southern General, told delegates how he had found a high level of abnormal proteins which are associated with head injuries and dementia while examining brain tissue of a former rugby player in his fifties. It is the first confirmed case of early-onset dementia caused by playing the game.
Protein abnormality levels he discovered exceeded those of dementia pugilistica - punch-drunk syndrome. Thought only to affect boxers who suffered repeated concussive injuries, it typically appears around 15 years after the start of a boxer's career. It is associated with impaired co-ordination, memory, speech, and personality problems.
Dr Stewart told the BBC the percentage of rugby players affected is "likely to be far lower than in boxing, American football, and ice hockey . . . but I think it would be foolish to think there will be no problem, and that rugby is immune from brain damage." He suggested that in any Six Nations weekend it was realistic that: "one or two players may go on and develop a dementia they wouldn't otherwise have been exposed to."
So far, so bad, but another ground-breaking study by Dr Stewart and his colleagues identified widespread Alzheimer-like pathology in a third of long-term survivors of a single head injury, even in young adults. This has devastating implications for all combat and high-impact sport, far beyond predictable American football, ice hockey, rugby, football, National Hunt racing, and boxing.
O'Driscoll suggests rugby could be hit by multi-million pound litigation, akin to that affecting the NFL in America. A decision in October by a court in Pennsylvania settled a class action involving 4500 former NFL players at $750m. Due for final approval within weeks, it opens the door to current players who have been told for years by NFL coaches to lead with their heads.
The US court heard evidence that players suffered a condition known as chronic traumatic encephalopathy (CTE), which can lead to loss of cognitive function, dementia, aggression and depression. They also considered mounting data on former players having higher rates of neurodegenerative diseases such as Alzheimer's and Parkinson's.
Last year, former linebacker Junior Seau, a gridiron legend, shot himself in the chest. He deliberately emulated former Chicago Bears' safety, Dave Duerson, who the previous year had committed identical suicide. Not in the head, because as he said in his note, his brain was to be donated to Boston University Medical School. They found he had CTE. Ditto Seau, and ditto Ray Easterling, formerly of the Atlanta Falcons. Another suicide, he was part of the class action and his estate is likely to be a beneficiary.
One of Seau's San Diego team mates told the Los Angeles Times that any linebacker who did not see stars at least five times during a game was not doing his job. "Players don't discuss it, fearing they will be replaced. Blocking out pain is taught to kids by coaches from Pop Warner up."
Pop Warner is the biggest and oldest US youth league, but parents are running scared. It has seen a 9.5% participation drop from 2010-12 - before the outcome of the $750m lawsuit.
A US study, published in the journal Neurology, compared the brains of 50 people who had suffered mild concussion with 50 healthy people. Initially there were memory problems, headaches and dizziness which resolved after a few weeks. But four months later, doctors observed differences in the way fluid moved through the brain which suggested it had still not healed. This supports O'Driscoll's attack on brief pitch-side assessment.
Rugby and football authorities ignore this at the peril of massive litigation. They are courting human and financial disaster.
How long will a gridiron helmet remain the most sought-after item of American kids? How long will parents support sporting dreams which threaten brain-damage, Alzheimer's, or Parkinson's?
Repeatedly heading a football? Suffice to say, recent research casts the case of the late Billy McPhail in a new light. He died in 2003, having lost a Scottish industrial tribunal at which he tried to claim disability benefit, on the grounds of his dementia being football-related. Since that verdict, a coroner has ruled that the death of England footballer Jeff Astle was due to a degenerative brain disease caused by heading heavy leather footballs causing: "repeated small traumas to the brain".
The Albert Einstein College of Medicine in New York has warned about the cumulative effects of heading a ball. Michael Lipton, associate director of the college's Magnetic Resonance Research Centre said: "Given that soccer is the most popular sport worldwide and is played extensively by children, these are findings that should be taken into consideration in order to protect soccer players."
Sportsmen are sometimes sidelined for months to allow damaged muscles to heal. These cells regenerate. Brain nerve cells do not. Any number of other potential factors, from grid and rugby's macho gym-monkey culture to nutrition supplements and dehydration at the time of injury, cannot be ignored.
Difficulty in concussion-diagnosis has been highlighted by Scotland guru Dr James Robson, among the world's most respected rugby medical authorities.
He acknowledges having allowed Scotland winger Simon Webster to remain on the pitch at Murrayfield after examining him. It was a further 15 minutes before Webster showed signs of dizziness, and Robson took him off. If that can happen with the expertise at international level, pity the club amateur especially where medics are not present.
We deplore the growing tendancy towards a nanny state, and are not advocating macrame on the back lawn for our kids, but contact sport as we know it needs critical reappraisal.