Concussion is a temporary injury to the brain caused by a bump, blow or jolt to the head.
It usually only lasts up to a few days or weeks, although it sometimes needs emergency treatment and some people can have longer-lasting problems.
Signs of a concussion usually appear within a few minutes or hours of a head injury.
But occasionally they may not be obvious for a few days, so it's important to look out for any problems in the days following a head injury.
Concussion can be harder to spot in babies and young children.
One of the main things to look for is a change in their normal behaviour after a head injury, such as crying a lot, a change in their feeding or sleeping habits, or a loss of interest in people or objects.
You don't usually need to get immediate medical advice if you only have mild symptoms that don't last long after a head injury, such as:
You probably don't have concussion, and can follow the advice about treating a minor head injury at home.
Call NHS 111 for advice if you're not sure if you need medical help.
Go to your nearest accident and emergency (A&E) department if you've injured your head and have:
In these cases, you should be checked by a health professional trained in assessing head injuries. They'll decide if you need a brain scan to rule out a serious brain injury.
Call 999 for an ambulance if someone has injured their head and has:
Also call for an ambulance if someone needs to go to hospital but you can't get them there safely.
If you're diagnosed with concussion in hospital, you'll be able to go home when any serious brain injury has been ruled out and you're starting to feel better.
Most people feel back to normal within a few days or weeks of going home. But some people, especially children, can take longer to recover.
Things you can do to help your recovery include:
Speak to your GP if you still have symptoms after 2 weeks or you're unsure about returning to activities such as work or sports.
Get medical help straight away if you develop any symptoms that mean you should go to hospital or call 999.
In some people, concussion symptoms can last a few months or more. This is known as post-concussion syndrome.
Possible symptoms include:
See your GP if you still have symptoms after 3 months. They may be able to recommend treatments for some of the symptoms, or they may refer you to a specialist.
The charity Headway has a leaflet on minor head injury and concussion (PDF, 303kb) that you might find useful if you're having long-term problems.
There's no guaranteed way to prevent concussion, but there are some simple things you can do that may reduce your risk of a head injury.
It's important to avoid head injuries as repeated concussions or blows to the head have been linked to serious problems, including a brain condition called chronic traumatic encephalopathy.
Read more about how to prevent head injuries.