Play-related head blows resembling concussions in American football have been shown to be the cause of chronic traumatic encephalopathy (CTE), which has led to player deaths and other debilitating symptoms after retirement, including memory loss, depression, anxiety, headaches, stress, and sleep disturbances.
The list of ex-NFL players that have either been diagnosed post-mortem with CTE or have reported symptoms of CTE continues to grow. According to Boston University, CTE is a brain degenerative disease found in athletes, military veterans, and others with a history of repetitive brain trauma. Although CTE is highly controversial and misunderstood, it is believed that tau proteins form clumps that slowly spread throughout the brain, killing brain cells.
There is also theoretical research that suggests early CTE might result from damaged blood vessels within the brain. That could trigger brain inflammation and, eventually, the development of proteins such as tau believed to play a key role in CTE. This hypothesis was tested on adult mice; the researchers state that their brains possess similar attributes to that of human brains. Using a special device, the mice were given precise impacts that would lead to mild brain traumas similar to what an athlete would suffer in contact sports. The mice, whose brains were scanned using specialized MRI, immediately showed changes to the electrical functions of their brains.
According to a 2017 study on brains of deceased gridiron football players, 99% of tested brains of NFL players, 88% of CFL players, 64% of semi-professional players, 91% of college football players, and 21% of high school football players had various stages of CTE.
Other common injuries include injuries of legs, arms, neck and lower back.
NFL concussions
The National Football League (NFL) and the National Football League Players Association (NFLPA) maintain procedures for identifying, evaluating and managing suspected concussions. The protocol includes preseason education and baseline neurological assessment, evaluation of suspected injuries during games, and a staged return to participation. During games, club medical staff work with unaffiliated neurotrauma consultants and athletic trainers monitoring play from the booth. Players with suspected concussions are removed for evaluation, and a player diagnosed with a concussion may not return to play on the same day.
Return to participation proceeds through five phases, beginning with symptom-limited activity and progressing to full football activity. There is no fixed recovery timetable; progression depends on the player's clinical assessment and response to activity. Before returning to full participation, the player must receive clearance from the club physician, with that decision confirmed by an independent neurological consultant.
In January 2025, the NFL reported that recorded concussions during the 2024 preseason and regular season, including games and practices, were 17% lower than in 2023. The league described the total as its lowest since the current electronic medical record system was introduced in 2015.
The NFL and NFLPA publish helmet rankings based on laboratory tests. The Centers for Disease Control and Prevention (CDC) states that helmets can reduce the risk of serious head injury, but no helmet can prevent every concussion.
Repeated head impacts include impacts that produce concussion symptoms and those that do not. According to the CDC, chronic traumatic encephalopathy (CTE) is associated with long-term exposure to repeated head impacts, but evidence is insufficient to conclude that occasional impacts or a history of one or more concussions alone lead to the disease. The agency notes that the symptoms directly attributable to CTE remain uncertain and that confirmation of the diagnosis requires examination of the brain after death.
A 2026 observational study of 6,201 NFL players examined documented concussion history and booking-based arrests using publicly available records. Players with a documented concussion history had higher odds of arrest in unadjusted analyses, but the association was not statistically significant after excluding cases in which the earliest arrest preceded the earliest documented concussion. The association with violent arrest was also not statistically significant. The authors characterized the findings as preliminary and cautioned against interpreting them as evidence of causation.
History
Concussions are frequent injuries among football players. Concussions occur when the head is subject to a large impact force, resulting in a minor brain injury. There has been a growing concern about concussions since the early 1900s. In 1906, a Harvard student-athlete died from a head injury and the team doctors released a report titled "The Physical Aspect of American Football" in the Boston Medical and Surgical Journal describing the type, severity, and number of injuries the team sustained in the 1905 season.
The NFL first began to review the subject formally in 1994, then NFL Commissioner Paul Tagliabue approved the creation of the Mild Traumatic Brain Injury (MTBI) Committee with the stated goal of studying the effects of concussions and sub-concussive injury in NFL players. Tagliabue appointed rheumatologist Elliot Pellman to chair the committee. Pellman's appointment was met with harsh criticism, because he is not a neurologist or neuropsychologist and often admitted ignorance about head injuries. The concussion data collected by the league from 1996 to 2001 has been shown to understate the actual number of diagnosed concussions by ten percent. The league additionally relied on legal counsel who were actively involved in downplaying health risks in the tobacco industry.Alan Schwarz, Walt Bogdanich and Jacqueline Williams. (March 24, 2016). "In NFL, Deeply Flawed Concussion Research and Ties to Big Tobacco " The New York Times. (New York). Retrieved March 24, 2016.
The same year, the National Institute for Occupational Safety and Health (NIOSH) reported a statistically significant increase in the risk of neurological disorders such as amyotrophic lateral sclerosis in retired football players, which furthered public knowledge about the risk of long-term neurocognitive disease related to repeated head impacts. Despite the NIOSH study, Pellman and the MTBI Committee drew their own conclusions that continued to contradict these findings and those of other organizations. Biomechanical engineers and neurosurgeons informed the Committee that the helmet safety standard at that time was insufficient to minimize the risk of concussions.
The MTBI Committee began studying the nature of tackle plays resulting in concussive impacts and developing its own biomechanical analysis of the effect of these forces on the brain. It started publishing study results in 2003 that stated there were no long-term negative health consequences associated with concussions sustained by NFL players. A six-year study by the Committee concluded that "Players who are concussed and return to the same game have fewer initial signs and symptoms than those removed from play. Return to play does not involve a significant risk of a second injury either in the same game or during the season."
Research Other organizations continued to publish study results that linked repeated concussions and long-term health problems contrary to reports by the MTBI Committee. A 2003 report by the Center for the Study of Retired Athletes at the University of North Carolina, for example, found a connection between numerous concussions and depression among former professional football players. Further, the center's follow-up study in 2005 associated both brain impairment and Alzheimer's disease with retired NFL players who had histories of concussions.
A 2004 doctoral dissertation by Don Brady examined NFL players' knowledge of concussions, studying both active and retired National Football League Players' knowledge of concussions. Brady's findings concluded that many NFL players lacked accurate and essential knowledge pertaining to various aspects of a concussion; that the preponderance of credible experimental and clinical evidence pertaining to the adverse effects of concussion indicates that the brain is injured as a result of a concussion; that the altered cell functioning and cell death along with subtle to more visible neurological, neurocognitive, psychological, and other medical problems reflect a diverse range of lifelong negative consequences of a concussion or brain injury; and that sports team health-care personnel need to focus primarily on the athletes' health and well-being, and not minimize an injury or primarily concentrate on the players' capacity to perform on the field. This expanded focus of health care is necessary in order to avoid any real or perceived conflicts of interest emerging in the concussion research, concussion management and related return to play decision-making process.
During November 2014, Brady filed objections to the proposed NFL concussion settlement offer. Brady sent a cover letter and detailed objections on behalf of NFL retired players to the presiding US district court judge, Anita Brody.
In addition to the studies that continued to contradict the work of the MTBI Committee, renowned experts and sports journalists wrote critical reviews of the committee's studies. Robert Cantu of the American College of Sports Medicine noted bias in the committee's extremely small sample size and held that no conclusions should be drawn from the NFL's studies. In an ESPN Magazine article titled "Doctor Yes," Peter Keating criticized Pellman and the MTBI Committee's work and argued that the "... Committee has drawn a number of important conclusions about head trauma and how to treat it that contradict the research and experiences of many other doctors who treat sports concussions, not to mention the players who have suffered them."
More studies continued to associate repetitive head injuries with neurological problems later in life. Kevin Guskiewicz, Director of the Center for the Study of Retired Athletes in the Department of Exercise and Sport Science at the University of North Carolina, analyzed data from a 2007 study of nearly 2,500 former NFL players. He found about 11 percent of the study participants suffered from clinical depression, with a threefold increased risk in former players who had a history of three or four concussions. The following year, the NFL commissioned the University of Michigan Institute for Social Research to conduct a study involving more than 1,000 former NFL players. The results reported that Alzheimer's disease or similar diseases appear to have been diagnosed in former NFL players vastly more often than in the general population at a rate of 19 times the normal rate for men ages 30 through 49. The NFL responded to these results by claiming the study was incomplete.
On September 30, 2014, researchers with Boston University announced that in autopsies of 79 brains of former NFL players, 76 had tested positive for CTE. As of January 2017, that number had grown to 90 out of 94. A study published in The Journal of the American Medical Association in July 2017 showed that 110 of 111 former NFL players whose brains were examined were found to have suffered from CTE. A 2026 study published in the British Medical Journal showed that of 338 former NFL players that donated their brains, 315 (93.2 percent) showed CTE pathology.
Later research examined the relationship between years of American football participation and CTE. A 2020 study reported that the odds of CTE doubled for every 2.6 additional years of football played, while the odds increased approximately tenfold for every 9 additional years of play.
Helmets