Brain fog: where it comes from and when it is worth getting tested

You walk into a room and forget why you came. You read the same part of a message several times. It is harder to find the right word, plan your day, or focus on a task that did not use to be a problem.

This is how many people describe brain fog.

It is not, however, a specific disease or a standalone diagnosis. The term "brain fog" describes a group of subjective difficulties related to concentration, memory, thinking speed, word finding, and organising everyday activities. Similar symptoms can appear in very different situations, from lack of sleep and chronic stress to vitamin B12 deficiency, thyroid problems, perimenopause, medication effects, or Long COVID.

That is why the most important question is not: "Which supplement is best for brain fog?", but: what, in this particular case, is making it harder for the nervous system to work properly?

For some people, better sleep, less overload, or a change in how work is organised is enough. Others need tests and treatment of a specific cause. Sudden thinking problems, disorientation, or trouble speaking, on the other hand, require urgent assessment and should not simply be labelled as brain fog.

What exactly is brain fog?

Brain fog can include:

• trouble holding attention,
• slower information processing,
• problems recalling words,
• forgetting recent conversations or arrangements,
• difficulty planning and organising,
• feeling overwhelmed by simple tasks,
• more mistakes,
• difficulty doing several things at once,
• a feeling of mental fatigue,
• the sense that thinking takes much more effort.

Symptoms can change during the day. They often worsen after a poor night's sleep, intense mental effort, stress, an infection, pain, or too many stimuli. In people with conditions that involve fatigue, problems with memory, attention, and processing speed can significantly interfere with studying, work, and relationships. Brain fog sometimes overlaps with chronic fatigue, which is also worth assessing.

Brain fog does not always mean a memory problem

Sometimes the problem is not storing information, but not registering it well enough.

If, during a conversation, you are at the same time checking your phone, thinking about the next task, and running on little sleep, some information may not be recorded accurately. Later you get the impression of poor memory, even though the original difficulty was attention.

That is why, during assessment, it is worth distinguishing:

The most common causes of brain fog

1. Lack of sleep and problems with its quality

Sleep is one of the first areas worth analysing. Cognitive difficulties can appear both with too little sleep and with frequently interrupted sleep.

You may spend eight hours in bed but actually sleep much less because of long sleep onset, night awakenings, pain, reflux, night sweats, or breathing problems.

Sleep apnoea may be suggested by:

• snoring,
• observed pauses in breathing,
• waking up with a feeling of choking,
• morning headaches,
• dry mouth,
• frequent urination at night,
• sleepiness and fatigue during the day,
• problems with concentration.

In women, apnoea may more often show up as fatigue, insomnia, headaches, low mood, and frequent awakenings, even without very loud snoring. The risk increases, among other times, during and after menopause.

2. Chronic stress, anxiety, and depression

The brain does not work in isolation from emotions.

Chronic tension can consume part of attention and working memory. A person worrying about several matters at once has fewer resources to focus on the current task.

Depression, in turn, can cause:

• slowed thinking,
• difficulty starting to act,
• problems with concentration,
• sleep disturbances,
• reduced motivation,
• a sense of mental exhaustion.

NICE points out that problems with memory and concentration, especially in younger adults without other neurological symptoms, may be related to stress, mood disorders, alcohol, and the effect of medicines or substances.

This does not mean that the symptoms are "all in your head". Mental health conditions can genuinely affect information processing speed, attention, and everyday functioning.

3. Perimenopause and menopause

Problems with concentration, memory, and word finding are often reported during perimenopause and after menopause.

Cognitive functioning may at the same time be affected by:

• fluctuations in sex hormones,
• night sweats and hot flushes,
• worse sleep,
• low mood,
• anxiety,
• being overloaded with duties,
• natural age-related changes.

A 2025 meta-analysis covering 24 studies and more than 5,600 women showed only a small association between subjective cognitive difficulties and objectively measured learning efficiency. In the other areas analysed, the associations were not significant. This means that perceived brain fog is a real experience, but does not always correspond to a marked drop in neuropsychological test results.

Menopausal symptoms should not automatically be interpreted as the onset of dementia. If, however, the problems are progressive, cause loss of independence, or clearly change everyday functioning, a broader assessment is needed.

4. Iron deficiency

Iron deficiency can occur even before anaemia develops.

Possible symptoms include:

• fatigue,
• worse concentration,
• memory problems,
• reduced exercise tolerance,
• headaches,
• palpitations,
• hair loss,
• restless legs syndrome.

The risk is higher with heavy periods, pregnancy, a diet low in iron, regular blood donation, gastrointestinal diseases, and absorption problems. A normal haemoglobin result alone does not rule out low iron stores, so with matching symptoms a doctor may consider measuring ferritin and transferrin saturation. You can read more about interpreting a basic blood test in the article on full blood count parameters.

You should not, however, take iron "just to try". Supplementation can cause side effects, and a confirmed deficiency requires establishing its cause.

5. Vitamin B12 deficiency

Vitamin B12 is needed for the nervous system to work properly and for blood cell formation.

Its deficiency can cause:

• fatigue,
• concentration problems,
• cognitive problems,
• tingling or numbness of the limbs,
• balance problems,
• weakness,
• pallor,
• inflammation of the tongue.

Neurological symptoms can appear even without anaemia. People at particular risk are those on a vegan diet without appropriate supplementation, older people, patients with pernicious anaemia, coeliac disease, bowel diseases, and those taking, over a long time, certain medicines that affect absorption.

B12 supplementation improves symptoms resulting from a deficiency. There is, however, no evidence that high doses improve cognitive function in every person with a normal nutritional status.

6. Thyroid disorders

Hypothyroidism can cause fatigue, low mood, slowing, trouble tolerating cold, constipation, dry skin, hair changes, and irregular or heavy periods.

Symptoms usually develop gradually and are non-specific. Fatigue or trouble concentrating alone does not yet mean thyroid disease, but with a broader clinical picture measuring TSH may be justified.

Hyperthyroidism, too, can indirectly worsen concentration through insomnia, palpitations, tension, and weakness.

7. Blood sugar problems and undernutrition

Long gaps between meals, a restrictive diet, or too little energy intake can lead to weakness, irritability, and trouble concentrating.

Similar symptoms can, however, also occur with abnormal blood sugar. The need to assess carbohydrate metabolism may be suggested by:

• increased thirst,
• frequent urination,
• sleepiness,
• recurrent infections,
• unexplained weight change,
• gestational diabetes in the history,
• diabetes in the family.

A basic assessment may include fasting glucose or HbA1c. Fasting insulin and HOMA-IR are not universal tests that explain brain fog.

8. Medicines and supplements

Some preparations can cause sleepiness, slowing, or memory problems.

Particular attention is paid, among others, to:

• sedatives and sleeping pills,
• benzodiazepines,
• opioids,
• some anti-allergy medicines,
• medicines with anticholinergic action,
• certain anti-epileptic medicines,
• a combination of several preparations with a sedative effect.

The National Institute on Aging recommends, when assessing cognitive problems, reviewing both prescription medicines and over-the-counter preparations. It is especially worth checking antihistamines, anticholinergics, sedatives, benzodiazepines, and opioid painkillers.

You should not stop medicines on your own. It is, however, worth checking whether the symptoms appeared after starting treatment, increasing the dose, or adding another preparation.

9. Long COVID

Problems with concentration, memory, information processing speed, and mental fatigue can occur after SARS-CoV-2 infection.

Long COVID can also appear after a mild or asymptomatic infection. Symptoms can change over time, subside, and flare up again. At present there is no single laboratory test that confirms or rules out Long COVID. Diagnosis is based on the history, examination, and targeted exclusion of other causes.

Longitudinal studies indicate that in many people cognitive function gradually improves, but problems with processing speed and executive function may persist for longer.

It is especially important to recognise post-exertional worsening, when symptoms intensify only after physical or mental activity and last for the following days. In such a situation, adding ever more intense training may not be the right first step.

10. Pain, chronic diseases, and cancer treatment

Chronic pain makes it harder to focus, worsens sleep, and uses up part of your attention resources. Similar problems can occur with fibromyalgia, ME/CFS, autoimmune diseases, and chronic infections.

Cognitive changes are also described during and after cancer treatment. They may be related to the disease itself, chemotherapy, radiotherapy, hormone therapy, anaemia, infection, pain, medicines, sleep problems, or treatment-induced menopause.

In these situations there is not always a single cause. The plan should take several overlapping elements into account.

When is it worth getting tested?

There is no single "brain fog panel" that would be right for everyone.

It is worth discussing tests with a doctor when the symptoms:

• persist or keep coming back,
• gradually intensify,
• clearly affect work, studying, or relationships,
• appeared without a noticeable lifestyle change,
• occur together with significant fatigue,
• accompany heavy periods,
• appear with numbness or balance problems,
• coincide with a weight change,
• began after an infection,
• appeared after a change in medicines,
• are also noticed by people close to you,
• make it harder to carry out everyday activities independently.

With progressive problems in memory, planning, or carrying out familiar activities, a medical assessment is needed regardless of age. A doctor may take into account how you function, your medicines, illnesses, mood, sleep, and short memory and attention tests.

Which tests may be considered?

The scope depends on the history. In practice a doctor may consider, among others:

Not everyone needs all the tests at once. Too broad a panel can detect incidental, unimportant abnormalities and lead to further unnecessary tests.

Which tests are not automatically needed?

Cortisol

An incidental cortisol measurement is not a basic test performed because of typical brain fog. This hormone has a clear daily rhythm, and diagnosing adrenal diseases requires a specific protocol.

Insulin and HOMA-IR

They are not universal tests for fatigue or concentration problems. In the basic assessment of carbohydrate metabolism, glucose and HbA1c are more important, and in specific situations an OGTT.

Broad vitamin panels

Not everyone needs a dozen or so vitamins and elements measured. Tests should follow from diet, symptoms, illnesses, and deficiency risk.

"Neurotransmitter" tests from blood or urine

They do not show, in a simple way, the activity of dopamine, serotonin, or other neurotransmitters in the brain, and are not a routine diagnostic tool for brain fog.

IgG intolerance tests

They do not identify the cause of cognitive problems and should not be used to create broad elimination diets.

What can you observe over two weeks?

Before your appointment it is worth keeping a short diary.

Record:

• your bedtime and wake time,
• the number of night awakenings,
• the moments of worst concentration,
• meals and long gaps in eating,
• the amount of caffeine and alcohol,
• the phase of your cycle,
• night sweats or hot flushes,
• your stress level,
• physical activity,
• medicines and supplements you take,
• infections in recent months,
• symptoms that appear together with the fog.

It is also helpful to write down specific examples. The information "I have brain fog" says less than "three times a week I forget about meetings, even though they are in my calendar" or "after an hour of work I can no longer read with understanding".

What can help day to day?

Take care of the basics of sleep

Set a wake-up time that is as regular as possible and give yourself a long enough sleep window. With snoring, night awakenings, or sleepiness, do not limit your action to "sleep hygiene" only.

Limit doing several tasks at once

Switching between messages, conversation, and work increases the load on working memory.

It may help to have:

• one task on the screen,
• notifications off,
• planned focus blocks,
• short breaks,
• the phone out of reach.

Use external memory

A notebook, calendar, reminders, and lists are not a sign of poor memory. They reduce the amount of information you have to keep in working memory at once.

UCLH recommends, among other things, writing information down straight away, using reminders, key words, lists, and cues that help with recall.

Match hard tasks to the time of day

If you function best in the morning, use that time for work that requires focus. Simple and routine tasks can be moved to the hours when concentration naturally drops.

Eat enough and regularly

Your diet does not have to be perfect, but it should provide an adequate amount of energy, protein, iron, vitamin B12, and other nutrients.

Add movement, but take tolerance into account

Regular activity can support sleep, mood, and cognitive functioning. With Long COVID or ME/CFS, however, you need to watch for post-exertional worsening and match activity to your current tolerance.

Do not treat brain fog with a set of supplements

Preparations with B12, iron, magnesium, ginseng, adaptogens, or "nootropics" will not solve every cause. Supplementation makes the most sense with a deficiency or a specific indication.

The National Institute on Aging recommends caution towards preparations promising improved memory and "sharpening the brain", because some may be ineffective, expensive, or interact with treatment.

The most common misconceptions

Summary

Brain fog is not a single disease. It is a term for difficulties with concentration, memory, thinking speed, word finding, and organising action.

There is no universal test panel or single supplement suitable for everyone. The scope of diagnostics should follow from the symptoms, age, periods, diet, illnesses, medicines, and how the problems affect everyday functioning.

In practice it is worth starting with an assessment of sleep, stress level, medicines, and diet. With persistent symptoms, a doctor may consider a full blood count, ferritin, TSH, vitamin B12, glucose, HbA1c, or other tests matched to the situation.

Sudden confusion, speech problems, weakness on one side of the body, seizures, or a very severe headache are not typical brain fog. They require urgent help.

This material is educational and does not replace an individual medical consultation. Progressive or sudden cognitive disturbances require appropriate medical assessment.

Frequently asked questions

Is brain fog a diagnosis?

No. It is a colloquial term for difficulties with attention, memory, thinking speed, and organisation. The diagnosis should concern the cause, for example sleep disorders, iron deficiency, thyroid disease, depression, perimenopause, or Long COVID.

Does brain fog mean dementia?

Most often not, especially when the symptoms change depending on sleep, stress, the cycle, or overload. Attention is needed, however, for problems that are progressive, are visible to those close to you, and affect independent performance of familiar activities, managing finances, orientation, or safety.

Is brain fog normal in perimenopause?

It is often reported, but not every problem should automatically be attributed to hormones. Night sweats, worse sleep, low mood, iron deficiency, and thyroid disorders can occur in the same period. If the symptoms strongly affect your functioning, it is worth discussing them with a doctor.

Do you need to test sex hormones?

Not always. In most women over 45, perimenopause is recognised mainly on the basis of changes in the cycle and symptoms. Single FSH and estradiol results can fluctuate strongly and do not explain all cognitive problems.

Does ferritin matter with normal haemoglobin?

It can. Iron deficiency without anaemia can be linked with fatigue, worse attention and memory, and restless legs syndrome. The result must, however, be interpreted in the context of symptoms, inflammation, and the cause of any iron loss.

Is it worth supplementing iron without tests?

No. Excess iron can be harmful, and supplements often cause constipation, nausea, or abdominal pain. First you should confirm the deficiency and establish its cause.

Can vitamin B12 cause brain fog?

Its deficiency can cause fatigue, cognitive problems, and neurological symptoms. It is especially worth considering testing with a plant-based diet without supplementation, tingling of the limbs, absorption problems, stomach or bowel diseases, and long-term use of medicines affecting B12.

Will a high dose of B12 improve concentration?

Not necessarily. Supplementation helps with a deficiency. In a person with a normal level, high doses have not been shown to routinely improve memory and prevent cognitive decline.

Is it worth testing TSH?

It may be justified if, together with the fog, there are fatigue, cold intolerance, constipation, dry skin, weight change, heavy periods, or thyroid disease in the history. These symptoms are, however, non-specific, so the history alone does not decide the diagnosis.

Does insulin resistance cause brain fog?

Blood sugar problems can affect energy and concentration, but brain fog is not a test for insulin resistance. With symptoms and risk factors it is worth starting with an assessment of glucose and HbA1c. Fasting insulin is not a universal test explaining cognitive problems.

Does magnesium help with brain fog?

It may help someone with a deficiency, but it is not a universal preparation for improving concentration. There is no basis to treat every case of thinking problems with magnesium without assessing sleep, diet, medicines, and health.

Can creatine help?

Studies suggest possible, small benefits for selected cognitive functions, especially memory and processing speed. This is not, however, a treatment for all causes of brain fog. Creatine will not replace sleep, treatment of iron deficiency, therapy for depression, or the diagnosis of neurological disorders.

Does coffee help?

Caffeine can temporarily increase alertness and reduce perceived sleepiness. If, however, it serves to mask chronic lack of sleep, it can sustain the problem. A large amount or a late time of consumption can worsen sleep quality and lead to another difficult morning.

Do phones and short videos cause brain fog?

Frequent interruptions of attention can make it harder to focus on a single task and increase the feeling of overload. This does not mean biological "dopamine damage". Limiting notifications, multitasking, and automatically reaching for the phone can help.

Can fog after COVID last many months?

Yes. In some people the symptoms persist for a long time, although in studies many cognitive functions gradually improved. There is no single test confirming Long COVID. The assessment should take into account the course of the infection, the remaining symptoms, and other possible causes.

With Long COVID, do you need to exercise more to regain fitness?

Not always. If exertion is followed by a delayed increase in fatigue, cognitive problems, pain, or other symptoms, activity has to be matched to tolerance. Pushing yourself can worsen the condition in some people.

Can brain fog result from undereating?

Yes. Long-term energy restriction, skipping meals, and insufficient intake of protein, iron, or vitamin B12 can affect energy and functioning. This is especially important during intense weight loss, training, or the use of medicines that greatly reduce appetite.

Does dehydration worsen concentration?

Insufficient fluid intake can contribute to headaches, weakness, and trouble concentrating. This does not mean, however, that every person has to drink a specific large amount of water. The requirement depends on temperature, diet, activity, illnesses, and medicines.

Is it worth having a head MRI?

It is not a routine test with typical, changeable brain fog without neurological symptoms. Imaging is considered on the basis of the medical examination, age, the pace of symptom development, and warning signs. A feeling of worse concentration alone is not an automatic indication for an MRI.

Do neuropsychological tests make sense?

They can be helpful when the symptoms are progressive, affect work or independence, or when it is necessary to determine precisely which area of cognitive function is impaired. Not everyone with a short-term drop in concentration after stress or lack of sleep needs a full neuropsychological work-up.

When should you see a neurologist?

It is worth considering a consultation with progressive cognitive problems, new neurological symptoms, balance problems, numbness, a change in speech, seizures, unusual headaches, or a significant worsening of everyday functioning. In younger adults with a normal short memory test and symptoms typical of trouble concentrating, a routine neurological referral is not always necessary.

Can brain fog go away on its own?

Yes, especially when it results from short-term stress, an infection, lack of sleep, or overload. If, however, the symptoms do not resolve, intensify, or affect functioning, it is not worth spending months trying more supplements without establishing the cause.

References

1. University College London Hospitals NHS Foundation Trust. Understanding and Managing Brain Fog. Current materials on cognitive symptoms related to fatigue.
2. National Institute for Health and Care Excellence. Suspected Neurological Conditions: Recognition and Referral. Guideline NG127, 2023 update.
3. Furey R.T., Thomas E.H.X., Kulkarni J., Gurvich C. Subjective versus Objective Cognition During Menopause: A Systematic Review and Meta-analysis. Journal of the International Neuropsychological Society. 2025;31:459–477.
4. Centers for Disease Control and Prevention. Long COVID Clinical Guidance. March 2026 update.
5. Becker J.H. et al. Neurocognitive Trajectories in Long COVID: Evidence from Longitudinal Analyses. Brain, Behavior, & Immunity – Health. 2025;48:101093.
6. National Institutes of Health, Office of Dietary Supplements. Vitamin B12: Fact Sheet for Health Professionals. Current materials on vitamin B12 deficiency.
7. National Institute of Diabetes and Digestive and Kidney Diseases. Hypothyroidism. Current materials on the symptoms and diagnosis of hypothyroidism.
8. Royal United Hospitals Bath NHS Foundation Trust. Ferritin: A Guide for GPs. Materials on iron deficiency without anaemia.
9. National Heart, Lung, and Blood Institute. Sleep Apnea and Women. 2025 update.
10. National Institute on Aging. Memory Problems, Forgetfulness, and Aging. Current materials on the assessment of cognitive problems.
11. National Institute on Aging. Caring for Older Patients with Cognitive Impairment. Recommendations on assessing the effect of medicines on cognitive function.
12. National Health Service. Sudden Confusion. Current materials on symptoms requiring urgent help.
13. Mayo Clinic. Chemo Brain: Symptoms and Causes. Current materials on cognitive disturbances related to cancer and its treatment.