Brain fog that won't go away: common causes and when to get help

Brain fog is one of those phrases everyone understands and nobody can quite define. It is not a medical diagnosis. It describes how thinking feels, and it has many possible causes, most of them common and many of them treatable.

This guide is for fog that has hung around: after an illness, after a knock to the head, or for no obvious reason at all. It covers what brain fog is, the usual causes, when to see a GP and where research into persistent symptoms after brain injury currently stands.

What does brain fog feel like?

Researchers in Edinburgh looked at how people describe brain fog in their own words, in posts on the social media site Reddit. In the study, published in the Journal of Neurology, Neurosurgery and Psychiatry, 141 first-person descriptions included:

• forgetfulness
• difficulty concentrating
• feeling detached or unreal
• thinking that feels slow or takes extra effort
• trouble communicating, such as finding words
• a fuzzy feeling or pressure in the head
• fatigue

That range is the point. Two people saying "brain fog" may be describing different experiences with different causes. Being specific about yours, such as losing words mid-sentence or rereading the same page, helps a doctor work out what is going on.

What causes brain fog that won't go away?

Most causes fall into a handful of groups, and more than one can apply at once.

Sleep. Poor or broken sleep is one of the most common explanations. The NHS lists sleeping problems among common, treatable causes of memory problems. It also says sleep apnoea, where breathing stops and starts during sleep, can leave you feeling very tired and finding it hard to concentrate.

Stress, anxiety and low mood. The NHS names stress, anxiety and depression as common causes of memory problems too. The physical symptoms of depression include moving or speaking more slowly than usual and a lack of energy, and it can make decisions feel difficult.

Menopause and perimenopause. The NHS lists problems with memory or concentration among menopause symptoms, and notes they may feel worse if you have sleep problems and feel very tired.

Thyroid and vitamin B12. An underactive thyroid can cause difficulty concentrating or thinking clearly, alongside tiredness and low mood. Vitamin B12 or folate deficiency can cause problems with memory, understanding and judgement, and the NHS warns that some problems caused by the condition can be irreversible if left untreated.

After COVID-19 or another infection. The NHS lists problems with memory and concentration, also called brain fog, as a symptom of long COVID. Long COVID is the term used when symptoms of COVID-19 last longer than 12 weeks.

After a head injury. Concussion can leave impaired concentration, memory problems and extreme tiredness that last for months. Headway notes these symptoms can also lead to anxiety and depression. NICE guidance says post-concussion syndrome is under-recognised after mild head injuries, and that a head injury of any severity can affect the pituitary gland, sometimes weeks or months later.

Medicines, alcohol and everyday load. A GP may ask about new or changed medicines, how much you drink, and how much you are carrying at work and at home. These are ordinary questions, and the answers often matter.

If you are wondering about blood tests, that is a conversation for your GP, who can decide what makes sense for your symptoms and history.

How long does brain fog last?

It depends on the cause. Fog from a run of bad nights can lift within days once sleep improves. Fog linked to an underactive thyroid or low B12 is unlikely to clear until the cause is found and treated. After a concussion, the NHS says symptoms can last up to 2 weeks, while NHS inform uses the term post-concussion syndrome once symptoms have lasted more than 28 days. Headway notes that even when problems persist for months, they can still resolve eventually.

What is not normal is fog that keeps getting worse, or confusion that arrives suddenly.

When should you see a GP about brain fog?

The NHS advice on memory problems is to see a GP if they are affecting your day-to-day life, and not to try to self-diagnose the cause. It suggests it can help to bring someone who can describe the problems you are having.

It is also worth booking an appointment if:

• the fog has lasted more than a few weeks and is not improving
• it started after a head injury and has lasted more than 2 weeks
• you have felt low most of the day, every day, for more than 2 weeks
• it comes with other changes, such as feeling tired all the time, loud snoring with pauses in breathing, or changes to your periods

Before the appointment, a simple note of when the fog is worse, how you are sleeping, what you drink and any medicines you take will make the conversation more useful.

Sudden confusion is different. The NHS says to go to A&E or call 999 if someone suddenly becomes confused, and not to drive yourself there. After a head injury, problems speaking, understanding, walking or staying awake also need 999.

What helps while you are waiting for answers?

None of these replace finding the cause, but they are sensible and low-risk:

• keep regular sleep and wake times
• cut back on alcohol
• break tasks into smaller pieces and write things down
• plan demanding work for the time of day you think most clearly
• rest before you hit a wall, rather than after
• keep some gentle movement in the day

If the fog followed a concussion, NHS inform's graduated return plan offers a useful structure: increase mental and physical effort in stages, and stay at the same stage if symptoms get worse.

Where does hyperbaric oxygen research fit?

Hyperbaric oxygen therapy (HBOT) is not a treatment for brain fog in general, and it is not a substitute for finding the cause. The research that exists looks at a specific group: people with persistent symptoms after a brain injury.

The most relevant recent study is a 2025 double-blind, sham-controlled trial. It analysed 47 adults aged 18 to 70 whose brain injury had happened between 6 months and 10 years earlier, most of them traumatic brain injuries. They had 40 sessions at 1.5 ATA, the pressure our chamber uses, or 40 sham sessions over 12 weeks. Symptom scores improved by 10.6 points in the HBOT group and 3.6 points with sham, and the HBOT group improved across cognitive, emotional and physical symptom areas.

The limits are significant. The trial reached only about a third of its planned size and was stopped when funding ran out. Both groups improved. An earlier trial in 72 US service members, also using 40 sessions at 1.5 ATA, found no benefit over sham. For long COVID the picture is contested too: the HOT-LoCO trial found no significant benefit from ten sessions.

So the honest position is that HBOT is a researched option for some people with long-lasting symptoms after a head injury, with mixed evidence and no promise of a result. If that describes you and your GP has looked at other causes, our head injury evidence page and who HBOT is not suitable for are the next things to read. When you are ready, you can see available times. No referral needed. Medical consultation included.