Written by Dr. Simon Khela MBChB MRCGP, GMC Registered Doctor
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A patient once came to see me three days after a minor fall at home, convinced he’d “gotten away with it” because he'd felt completely fine on the day. It was only once the fogginess, irritability and poor concentration set in over the following 48 hours that he realised something was actually wrong. That delay is one of the most common sources of confusion I see with concussion, and it's exactly why this guide spends so much time on timing, not just symptoms.
This article explains what concussion actually is, what causes it, the symptoms to recognise, how long they typically take to appear and to resolve, and, most importantly, the specific warning signs that mean a head injury needs emergency assessment rather than watching and waiting.
Concussion is a temporary disturbance of brain function caused by a blow, bump or jolt to the head, or a forceful movement of the head or body. It's a mild form of traumatic brain injury, and only around 1 in 10 concussions involve losing consciousness; you don't need to have been “knocked out” to have one. Symptoms can appear immediately or be delayed by hours or even days, commonly include headache, dizziness, confusion and difficulty concentrating, and most people recover fully within two weeks. A smaller number of people have symptoms that last longer, sometimes for months, which is known as post-concussion syndrome.
A concussion happens when an impact causes the brain to move suddenly within the skull. Common causes include:
A question I am frequently asked is whether you have to be knocked out for it to “count” as a concussion. You don't; loss of consciousness happens in only around 1 in 10 cases, so relying on this as the marker of whether something is serious is genuinely misleading.
Symptoms vary from person to person, but commonly include:
“Mild” refers to how concussion is classified alongside more severe traumatic brain injuries, not a guarantee that it will feel minor or resolve quickly. Mild concussion symptoms often include a mild headache, brief grogginess, slightly blurred vision, or feeling “not quite right” without any dramatic signs. Many patients are surprised to learn that these subtler symptoms deserve just as much attention as more obvious ones; the label “mild” describes the injury’s severity grade, not how seriously it should be taken.
Symptoms can begin immediately, but it's genuinely common for them to take up to 24 to 48 hours to fully develop. This is exactly why anyone with a head injury is generally advised to have someone stay with them for the first 24 hours, not because something is expected to go wrong, but because concussion is what's sometimes called an “evolving injury,” and early signs can be subtle.
Delayed concussion symptoms are common, and can include:
One of the most common concerns I see in practice is a patient who assumed they'd “escaped” a head injury because they felt fine on the day, only to develop noticeable symptoms 24 to 48 hours later. Headway, the UK brain injury charity, describes concussion as an evolving injury for exactly this reason; the symptoms genuinely can lag behind the injury itself.
Call 999 or go to A&E immediately if someone has hit their head and:
Get advice from NHS 111 if someone has recently had a head injury and:
If you're told to go to A&E, don't drive yourself there if you've hit your head; arrange for someone else to take you, or call an ambulance.
For most people, symptoms settle within about two weeks, though it's genuinely common for some tiredness or difficulty concentrating to linger a little longer than that. A smaller number of people take several weeks to feel completely back to normal.
Typical delayed onset is measured in hours to a day or two, not weeks, but there are rare exceptions worth knowing about. Head injury can very occasionally cause pituitary gland dysfunction that presents weeks or months later with a range of symptoms, and older adults or people on blood-thinning medication have a small but genuine risk of a slow bleed inside the skull developing gradually over days to weeks. These are uncommon, but they're exactly why persistent or new symptoms after a head injury, at any point, deserve proper assessment rather than being assumed to be “just concussion” indefinitely.
When symptoms continue beyond the expected few weeks, this is known as post-concussion syndrome (PCS). Research reviewed by Headway suggests that roughly 1 in 3 adults report at least mild ongoing symptoms 3 to 6 months after a mild traumatic brain injury, though this varies depending on how strictly PCS is defined. Symptoms can include:
Many patients are surprised to learn that PCS doesn't seem to be linked to how severe the original injury felt; some people with an apparently minor knock go on to develop prolonged symptoms, while others with a more dramatic-seeming injury recover quickly. If symptoms are still present after two to four weeks, it's genuinely worth being reassessed rather than assuming there's nothing more to be done.
Concussion is diagnosed clinically, based on the story of the injury and your symptoms, rather than through a specific scan. A CT scan isn't routinely needed for every head injury; it's generally reserved for people with specific risk factors or red-flag symptoms, in line with national guidance.
This approach is set out in NICE guidance on head injury assessment, which helps doctors decide who needs imaging and who can be safely monitored without it. If symptoms are unusual, persistent, or don't fit the expected pattern, a referral to a specialist, typically neurology, allows more detailed assessment.
Recovery should be gradual rather than all-or-nothing. Current UK concussion guidelines for grassroots sport recommend avoiding any return to contact sport for a minimum of 21 days, and only once you've been completely symptom-free for at least 14 days beforehand. The guiding principle, “if in doubt, sit them out,” applies just as much to adults as to children.
This is a general guide rather than a fixed rule; recovery genuinely varies between individuals, and the right pace is the one your own symptoms allow.
Beyond the emergency red flags above, it's worth seeing a GP if:
A private GP appointment is a reasonable way to have persistent symptoms properly assessed, and for straightforward follow-up once you're past the initial injury, an online GP consultation can often be arranged without needing to travel.
Concussion is a temporary disturbance of brain function caused by a blow, bump or jolt to the head, or a forceful movement of the head or body. It's a mild form of traumatic brain injury.
Common symptoms include headache, dizziness, nausea, confusion, difficulty concentrating, sensitivity to light or noise, irritability and disturbed sleep.
Mild concussion symptoms often include a mild headache, brief grogginess, slightly blurred vision, or a general feeling of not being quite right, without any dramatic or obvious signs.
Symptoms can start immediately, but it's common for them to take up to 24 to 48 hours to fully develop, which is why someone should stay with you for the first day after a head injury.
Yes. Concussion is sometimes described as an evolving injury, and delayed symptoms such as concentration problems, mood changes or sleep disturbance are common in the days after the injury.
Typical delayed onset is within 24 to 48 hours. Rare complications, such as pituitary dysfunction or a slow bleed inside the skull, can occasionally cause new symptoms weeks later, which is why persistent or new symptoms should always be checked.
Most people recover within about two weeks. Some people take several weeks, and a smaller number develop post-concussion syndrome, with symptoms persisting for months.
Post-concussion syndrome describes symptoms, such as headaches, dizziness, low mood and concentration problems, that continue well beyond the typical few weeks of concussion recovery.
Go to A&E or call 999 if someone has lost consciousness and hasn't woken up, can't stay awake, has a seizure, has new weakness or numbness, or shows a significant change in behaviour, among other red-flag symptoms.
Current UK guidance recommends waiting a minimum of 21 days before returning to contact sport, and only once you've been completely symptom-free for at least 14 days.
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