Concussion

Quick answer

A concussion is a type of mild traumatic brain injury (TBI) caused by a bump, blow or jolt to the head, or by a hit to the body that makes the head and brain move quickly back and forth.[1] Most TBIs that happen each year are mild TBIs or concussions.[1] According to the CDC, symptoms generally improve over time and most people feel better within a couple of weeks,[2] though some people have symptoms for months or longer.[3]

What a concussion is

A traumatic brain injury is an injury that affects how the brain works.[1] MedlinePlus, the National Library of Medicine’s consumer health service, groups TBIs into three levels: mild TBI (also called a concussion), moderate TBI and severe TBI.[4]

The National Institute of Neurological Disorders and Stroke (NINDS) describes a concussion as “a type of mild TBI that may be considered a temporary injury to the brain but could take minutes to several months to heal.”[5] NINDS also notes that a person with a concussion either suddenly loses consciousness or has a sudden change in their state of consciousness or awareness.[5]

Being knocked out is not required. MedlinePlus states that mild TBIs do not always lead to loss of consciousness and that most people never pass out; some describe seeing all white, all black or stars, and a person can have a mild TBI without realizing it.[4]

What happens in the brain

The CDC explains that the sudden movement behind a concussion can cause the brain to bounce around or twist in the skull, create chemical changes in the brain, and stretch and damage brain cells.[1] These changes lead to symptoms that may affect how a person thinks, learns, feels, acts and sleeps.[1]

Because the injury is mostly to how brain cells function rather than to the brain’s structure, standard imaging often looks normal. NINDS notes that currently available and clinically validated imaging, blood tests and other measures “can’t always detect damage from mild, concussive injuries.”[5]

Causes

Any force that moves the brain rapidly inside the skull can cause a concussion. NINDS lists a blow to the head or a rapid movement of the brain within the skull, which can happen when a person is violently shaken.[5] MedlinePlus lists falls, sports injuries, vehicle crashes and assaults among the common causes of TBI.[4]

The head does not have to be struck directly. The CDC includes “a hit to the body that causes the head and brain to move quickly back and forth” as a cause of mild TBI.[1]

Related pages: Car crashes · Falls · Sports concussions · Coup-contrecoup injury

Symptoms

Concussion symptoms vary from person to person and can change during recovery.[6] The CDC notes that some symptoms appear right away, while others may not appear for hours or days after the injury.[2] In its information on children, the CDC says symptoms are usually most severe in the first 1 to 2 days after the injury.[6]

The CDC groups concussion symptoms into four categories:[6]

Physical

  • Headache
  • Dizziness or balance problems
  • Nausea or vomiting (early on)
  • Sensitivity to light or noise
  • Feeling tired, no energy
  • Vision problems

Thinking and memory

  • Trouble concentrating
  • Feeling slowed down
  • Feeling foggy or groggy
  • Short- or long-term memory problems
  • Trouble thinking clearly

Emotional

  • Anxiety or nervousness
  • Irritability
  • Feeling more emotional
  • Sadness

Sleep

  • Sleeping more than usual
  • Sleeping less than usual
  • Trouble falling asleep

NINDS adds that headache, dizziness, confusion and fatigue tend to start right after the injury and fade over time, while emotional symptoms such as frustration and irritability tend to develop during recovery.[5] Other symptoms NINDS lists include ringing in the ears, an unexplained bad taste in the mouth, and changes in sleep patterns.[5] MedlinePlus also lists memory loss for events just before or right after the injury and a feeling of having “lost time.”[4]

Signs of a more serious injury

The CDC notes that it can be hard to tell the difference between a concussion and a more life-threatening brain injury.[6] It identifies the following as danger signs that call for 911 or an emergency department:[6]

  • A headache that gets worse and does not go away
  • Repeated nausea or vomiting
  • Convulsions or seizures
  • Slurred speech, weakness, numbness or decreased coordination
  • One pupil larger than the other, or double vision
  • Unusual behavior, increased confusion, restlessness or agitation
  • Not being able to recognize people or places
  • Loss of consciousness with increasing drowsiness, or being unable to wake up or stay awake

For infants and toddlers, the CDC adds crying that cannot be consoled and refusing to nurse or eat.[6] These signs can point to bleeding or swelling in or around the brain. See Brain bleeds and Skull fractures.

How concussions are diagnosed

There is no single test for concussion. NINDS describes a neurological exam that checks motor and sensory skills, hearing and speech, coordination and balance, mental status, and changes in mood or behavior.[5]

Imaging

According to NINDS, CT scans can show a skull fracture and any bruising, bleeding or swelling of the brain, while MRI is more sensitive and can pick up subtler changes a CT scan may miss.[5] A physical exam and the person’s symptoms help determine whether imaging is needed.[5] A normal scan does not rule out a concussion, because standard tests can’t always detect mild injuries.[5]

Blood tests

In 2018 the FDA cleared the first blood test for two brain proteins, GFAP and UCH-L1, to help evaluate adults with suspected mild TBI.[7] Several faster versions have been cleared since.[7] These tests do not diagnose concussion. They help clinicians predict which adult patients are unlikely to have bleeding or other lesions visible on a CT scan, and so may not need one.[7][8] They are cleared for use within 12 hours of injury in adults 18 and older.[8]

Thinking and memory tests

NINDS notes that neuropsychological tests are often used along with imaging after a mild TBI. These tests measure memory, concentration, information processing, reaction time and problem-solving.[5] In sports, clinicians use standardized sideline tools such as the SCAT6 and Child SCAT6, which the 2023 international consensus statement recommends repeating over the hours and days after an injury.[9]

Treatment

MedlinePlus states that a mild brain injury may need no treatment beyond monitoring, and that providers typically explain what to expect, how to manage headaches and other symptoms, and when to return to sports, school and work.[4] NINDS notes that some people with a concussion may need only rest and over-the-counter pain relievers, with treatment focused on symptom relief.[5]

Rest, then gradual activity

Medical guidance on rest has changed. The 2023 consensus statement from the Concussion in Sport Group found that strict rest until all symptoms are gone is not beneficial after a sport-related concussion.[9] It describes relative rest, meaning ordinary daily activities and reduced screen time, for up to the first two days, followed by light activity such as walking, as long as it does not more than mildly worsen symptoms.[9] The CDC’s adult guidance similarly describes easing back into regular activities after one or two days of rest, even with some mild symptoms.[3]

The consensus statement also found that reducing screen time in the first 48 hours appears to help, but may not be effective beyond that.[9]

Treatment for lasting symptoms

When specific symptoms persist, treatment targets them. Depending on the symptoms, this can include vestibular therapy for dizziness and balance, vision therapy, physical therapy for neck-related headaches, cognitive rehabilitation, and treatment for mood or sleep problems. See Vestibular therapy, Cognitive rehabilitation and Post-concussion syndrome.

Recovery

The CDC states that with proper care, most people can return to work, school and many other activities within a few days or weeks after a mild TBI or concussion.[3] NINDS reports that most people recover fully from a first concussion within a few weeks, but recovery from a second or third concussion is generally slower.[5]

The CDC defines recovery as being able to do regular activities without symptoms from the injury.[2] It notes that recovery may be slower among older adults, young children and people who have had a concussion or other TBI before.[3] The CDC advises talking with a healthcare provider if symptoms don’t go away within 2 to 3 weeks or get worse after returning to regular activities.[3]

Complications and long-term effects

Post-concussion syndrome

NINDS describes post-concussion syndrome as symptoms that last for weeks or longer.[5] The CDC notes that if one or more symptoms last for months after the injury, a provider may discuss post-concussive syndrome.[3] See Post-concussion syndrome.

Second impact syndrome

According to NINDS, a second concussion soon after the first, sometimes called the “second hit” phenomenon, can lead to permanent damage or even death in some cases.[5] This risk is a main reason every U.S. state has a youth sports concussion law requiring removal from play.[10] See Second impact syndrome.

Repeated concussions

The CDC states that repeated mild TBIs or concussions may be associated with ongoing problems with concentration, memory, headache and balance.[1] MedlinePlus notes that the risk of long-term changes in the brain is higher after more than one TBI.[4] NINDS reports that repeated TBIs may cause chronic traumatic encephalopathy (CTE), which develops over years.[5] See CTE.

Children, athletes and older adults

Children

Young children may not be able to describe their symptoms. The CDC lists signs in children up to age 4 such as crying more than usual, seeking more comfort, refusing to nurse or eat, changes in sleep, and losing interest in play.[6] For school, the 2023 consensus statement advises against complete rest and isolation and describes a stepwise return to learning that begins after 24 to 48 hours of relative rest.[9]

Athletes

Every state and the District of Columbia has a youth sports concussion law, though the requirements vary.[10] Return to sport follows a step-by-step progression. The BMJ’s summary of the 2023 consensus notes that full sports participation usually occurs within one month of injury.[11] See Sports concussions and your state’s concussion law.

Older adults

The CDC notes that recovery may be slower in older adults.[3] Its caregiver guidance says older adults should be checked by a healthcare provider for signs of TBI after a fall or car crash.[12] See Subdural hematoma.

If someone else caused the injury

Concussions from car crashes, falls on unsafe property, workplace accidents and assaults can lead to insurance or injury claims. Because concussions often don’t show up on scans, records of symptoms and treatment matter. See Head injury claims and Proving a mild TBI.

Frequently asked questions

Is a concussion a brain injury?

Yes. Both the CDC and NINDS classify a concussion as a mild traumatic brain injury.[1][5] “Mild” refers to how it is classified at the time of injury, not to how much it affects a person’s life.

Do you have to lose consciousness to have a concussion?

No. MedlinePlus states that most people with a mild TBI never pass out.[4]

Will a concussion show up on a CT scan or MRI?

Often not. NINDS notes that current imaging and other tests can’t always detect damage from mild, concussive injuries.[5] CT is mainly used to look for bleeding, swelling or fractures.[5]

How long does a concussion last?

The CDC says most people feel better within a couple of weeks,[2] but some people have symptoms for months or longer.[3] Recovery tends to be slower after repeat concussions.[5]

Can symptoms start days after the injury?

Yes. The CDC notes that some symptoms may not appear for hours or days.[2] MedlinePlus notes some TBI symptoms may show up a few days or weeks later.[4]

Is total rest in a dark room still recommended?

Not according to current consensus. The 2023 international consensus statement found strict rest is not beneficial and recommends relative rest for about the first two days, followed by gradual light activity.[9]

This page provides general information from the sources listed below. It is not medical advice and is not a substitute for evaluation by a qualified healthcare provider. In an emergency, call 911.

Sources

  1. Centers for Disease Control and Prevention. “About Mild TBI and Concussion.” https://www.cdc.gov/traumatic-brain-injury/about/index.html
  2. Centers for Disease Control and Prevention. “Symptoms of Mild TBI and Concussion.” https://www.cdc.gov/traumatic-brain-injury/signs-symptoms/index.html
  3. Centers for Disease Control and Prevention. “What to Do After a Mild TBI or Concussion.” https://www.cdc.gov/traumatic-brain-injury/response/index.html
  4. MedlinePlus Medical Encyclopedia, U.S. National Library of Medicine. “Traumatic brain injury.” https://medlineplus.gov/ency/article/000799.htm
  5. National Institute of Neurological Disorders and Stroke. “Traumatic Brain Injury (TBI).” https://www.ninds.nih.gov/health-information/disorders/traumatic-brain-injury-tbi
  6. Centers for Disease Control and Prevention, HEADS UP. “Signs and Symptoms of Concussion.” https://www.cdc.gov/heads-up/signs-symptoms/index.html
  7. JAMA Network Open. “Performance of Diagnostic Biomarkers for TBI Within the First Hour” (invited commentary). https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2823152
  8. National Institute of Neurological Disorders and Stroke. “Blood-based Biomarkers for TBI Classification” (working group report). https://www.ninds.nih.gov/sites/default/files/documents/blood-based-biomarkers-tbi-wg-508c.pdf
  9. Patricios JS, et al. “Consensus statement on concussion in sport: the 6th International Conference on Concussion in Sport–Amsterdam, October 2022.” British Journal of Sports Medicine, 2023. https://bjsm.bmj.com/content/57/11/695
  10. National Federation of State High School Associations. “Legal Perspectives, Recommendations on State Concussion Laws.” https://nfhs.org/stories/legal-perspectives-recommendations-on-state-concussion-laws
  11. BMJ Newsroom. “International expert panel revises management of concussion in sport for athletes at all levels.” https://www.bmj.com/company/newsroom/international-expert-panel-revises-management-of-concussion-in-sport-for-athletes-at-all-levels/
  12. Centers for Disease Control and Prevention, Still Going Strong. “Information for Caregivers.” https://www.cdc.gov/still-going-strong/caregivers/index.html

Sources last checked: October 11, 2026

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