What causes brain fog? The common reasons, how to get rid of it, and the tests that check them

Brain fog usually comes from poor sleep, stress, low mood, a medicine or hormone changes, and a short list of blood tests checks the medical causes worth ruling out.

Reviewed by LifeFrom’s medical AIPublished November 11, 2026 · 12 min read
Ask AI about this pageChatGPTClaudePerplexity
The distinctions
  1. 01
    Brain fog creeps in; sudden confusion is an emergency

    Brain fog builds over days or weeks and moves with sleep, stress, hormones and illness. Confusion that starts suddenly, especially with trouble speaking, a drooping face, weakness on one side, or fever and a stiff neck, is a different thing and needs 911.

  2. 02
    Brain fog is a description, not a diagnosis

    The phrase covers trouble focusing, forgetfulness and slow thinking from many causes. Finding which cause is behind it (sleep, mood, a medicine, hormones, thyroid, B12, iron, blood sugar) is what makes it lift.

Brain fog is the everyday name for trouble focusing, forgetfulness, slow thinking and losing your train of thought, and it most often comes from too little sleep, stress, anxiety or depression, a sedating medicine, or hormone changes such as perimenopause. 124 Long COVID is another cause. 3 The medical causes a blood test can check are an underactive thyroid, low vitamin B12, iron deficiency and anemia (too few healthy red blood cells), blood sugar problems and, less often, low blood sodium. 67810 Brain fog that creeps in over weeks is usually not an emergency; confusion that starts suddenly is.

When to get help now

Most brain fog is not dangerous. These signs are the exceptions.

Call 911 or your local emergency number
  • Sudden confusion, trouble speaking or understanding speech 11
  • A drooping face, or sudden weakness or numbness in an arm or leg, especially on one side 11
  • Confusion with fever, headache and a stiff neck 12
  • After a blow to the head: a worsening headache, repeated vomiting, growing confusion, slurred speech, a seizure, or being hard to wake 18
  • Shaking, sweating or confusion in someone on insulin or some diabetes pills who is too confused to treat it, or does not improve with sugar; severe low blood sugar needs treatment right away 9
  • Thoughts of harming yourself: call or text 988, the Suicide & Crisis Lifeline 5
See a doctor soon, the same day if you can
  • Memory or thinking that keeps getting worse over weeks or months, or that others notice 19
  • Brain fog after a blow to the head, even without the danger signs 18
  • Headache, nausea or confusion in someone on water pills or with heart failure, which can be low sodium; a seizure is a 911 call 10
  • Numbness or tingling in the hands or feet with the fog 7
  • Fog with weight loss you can't explain, fevers or new symptoms that keep coming 20
  • Feeling low or hopeless most days 5

LifeFrom can't examine you or assess a symptom. If something feels seriously wrong, trust that and get care rather than waiting on a test.

What is brain fog, and what isn't it?

Cleveland Clinic calls brain fog “a term for a range of symptoms” that affect thinking, and lists difficulty concentrating, forgetfulness, “Losing your train of thought,” “Not having the right words,” mental exhaustion and slow reactions. 1 It is a description, not a diagnosis, and everyone has foggy days after a bad night. What matters is how it started and what comes with it.

The distinction to hold on to is speed. Brain fog creeps in over days or weeks and moves with sleep, stress, hormones and illness. Confusion that arrives within minutes or hours is different: the CDC lists “Sudden confusion, trouble speaking, or difficulty understanding speech” as a stroke sign, and says to call 911 right away. 11 Confusion with fever, headache and a stiff neck can be meningitis. 12

Brain fog or sudden confusion: which is it?

Brain fogBook a visit if it lasts or worsens
Builds gradually
How it startsOver days or weeks
What it feels likeSlow, forgetful, hard to focus, still yourself 1
Moves withSleep, stress, hormones, illness 12
Who helpsYour clinician, sometimes with blood tests
Sudden confusionCall 911 now
Starts within minutes or hours
How it startsSuddenly 11
What it feels likeDisoriented, words come out wrong, can't follow speech 11
Moves withFace drooping, arm weakness, fever and stiff neck 1112
Who helpsEmergency services; an ambulance can start treatment on the way 11
Stroke signs from the CDC; meningitis signs from the CDC. 1112

What can cause it, most likely first

Ranked from most to least likely in adults in general. The order shifts with age, sex and history: perimenopause moves up for women in their 40s and 50s, medicines and low B12 for older adults, and long COVID for anyone who has had COVID-19.

  1. 01
    Too little sleep, or poor sleepEverydaycommon

    Sleep deficiency “can cause problems with learning, focusing, and reacting,” and with making decisions, solving problems and remembering things. 2 Cleveland Clinic lists a lack of sleep first among the causes of brain fog. 1 Snoring, gasping in sleep and daytime sleepiness are worth raising; see can't sleep.

  2. 02
    Stress, anxiety and depressionMedicalcommon

    Stress, anxiety and depression are on the Cleveland Clinic list of brain fog causes. 1 MedlinePlus lists “Trouble concentrating,” fatigue and sleeping too much or too little among the signs of depression. 5 No blood test checks this; a conversation with a clinician does. If low mood comes with thoughts of harming yourself, call or text 988. 5

  3. 03
    Medicines, especially antihistamines and sleep aidsEverydaycommon

    Diphenhydramine, the antihistamine in many allergy and night-time sleep products, causes drowsiness, and MedlinePlus says it generally “should not be used in older adults” except for serious allergic reactions. 13 It belongs to a group called anticholinergic medicines; in a study of 3,434 adults aged 65 and older, higher long-term use of strong anticholinergics was linked with a higher risk of dementia. 14 Do not stop a prescribed medicine on your own; ask the prescriber whether one could be adding to the fog.

  4. 04
    Perimenopause and menopauseMedicalcommon

    Menopause usually arrives between 45 and 55, and MedlinePlus lists forgetfulness in some women, sleep problems and mood swings among its symptoms. 15 In the SWAN study, perimenopause was linked with a dip in thinking-test performance that “rebounded to premenopausal levels in postmenopause.” 4 Night sweats that break up sleep add to it; see night sweats. Menopause is usually recognized from age and periods; hormone tests can support it. 15

    FSH
    A high FSH (follicle stimulating hormone) fits menopause; while periods continue it swings, so a provider may repeat it several times. 15
    Estradiol
    A low estradiol with a high FSH fits menopause; one result while periods continue says little. 15
  5. 05
    Long COVID or another lingering infectionMedicalless common

    The CDC lists “Difficulty thinking or concentrating” (sometimes called brain fog), tiredness that interferes with daily life, sleep problems and symptoms that get worse after physical or mental effort among the signs of long COVID. 3 More than 200 long COVID symptoms have been identified, and “There is no laboratory test that can determine if your symptoms are due to COVID-19 illness.” 3 See fatigue for the overlap with post-viral tiredness.

  6. 06
    Underactive thyroidMedicalless common

    The American Thyroid Association lists feeling tired, feeling cold and the tendency to “Forget things easily” among the signs of an underactive thyroid (hypothyroidism). 8 Other health problems can cause the same signs, so a blood test is how it is found. 8

    Can be linked with Hypothyroidism

    TSH
    A high TSH (thyroid stimulating hormone) points to an underactive thyroid: “If your thyroid is not making enough hormone, your TSH goes up.” 8
    Free T4
    A low free T4 with a high TSH fits an underactive thyroid. See how to read your thyroid panel.
  7. 07
    Low vitamin B12Medicalless common

    MedlinePlus lists problems concentrating, irritability, numbness and tingling, and in severe cases confusion, among the symptoms of anemia due to low B12. 7 The risk rises with long-term metformin, acid-reducing medicines, a strict vegetarian diet and stomach or weight-loss surgery. 717

    Can be linked with B12 deficiency

    Vitamin B12
    A B12 under 150 pg/mL is diagnostic of deficiency; a normal or low-normal result with symptoms leads to a methylmalonic acid test. 17
    Methylmalonic acid
    A high methylmalonic acid (MMA) points to B12 deficiency at the tissue level and is the next step when B12 is borderline. 17
  8. 08
    Iron deficiency and anemiaMedicalless common

    MedlinePlus lists “Problems concentrating or thinking” among the early symptoms of iron deficiency anemia, with tiredness, headaches and dizziness. 6 Heavy periods, bleeding in the gut and pregnancy are common reasons for low iron. 6 A clinician decides on iron after a test and looks for the cause.

    Can be linked with Iron deficiency

    Ferritin
    Low ferritin points to low iron stores; inflammation can push it up and hide a deficiency. See iron deficiency.
    Hemoglobin
    Low hemoglobin means anemia; see the CBC explainer and anemia.
  9. 09
    Blood sugar problemsMedicalless common

    Cleveland Clinic lists diabetes and low blood sugar (hypoglycemia) among the causes of brain fog. 1 In people taking insulin or some diabetes pills, low blood sugar can make you “dizzy, lightheaded, confused, or irritable,” and for many people with diabetes low means under 70 mg/dL. 9 See blood sugar explained.

    Can be linked with Prediabetes

    HbA1c
    A high HbA1c points to prediabetes or diabetes; it reads the average over recent months, so it can look normal while lows or swings happen day to day.
    Fasting glucose
    A high fasting glucose points to prediabetes or diabetes; a home meter or continuous monitor is what catches lows. 9
  10. 10
    Low blood sodiumMedicalrare

    Low blood sodium (hyponatremia) can cause confusion, headache, nausea and, when severe, seizures; MedlinePlus warns that a big drop “can be a life-threatening emergency.” 10 Diuretics (water pills), heart failure and conditions where the body holds on to water are among the causes. 10 See electrolytes explained.

    Sodium
    A sodium below the normal range of about 136 to 144 mEq/L points to low blood sodium; a sudden or large drop is the dangerous kind. 10

These are things a symptom like this can be linked with, not a diagnosis. Several can sit together, and a clinician who knows your history puts them in order for you.

How do you get rid of brain fog?

Brain fog lifts when its cause is found and dealt with, so the first job is to narrow it down. Start with sleep: adults who sleep too little have trouble focusing, deciding and remembering, and a week of earlier nights often tells you a lot. 2 Move most days; the CDC says “Regular physical activity can help keep your thinking, learning, and judgment skills sharp as you age,” and it helps sleep and mood too. 16 Look at the medicine cabinet, especially night-time cold and sleep products containing diphenhydramine. 13 If the fog persists after that, the blood tests below check the medical causes.

Brain fog with fatigue, or with feeling cold

Brain fog rarely travels alone. Fog with tiredness points to the shared causes: sleep, low mood, anemia, an underactive thyroid and long COVID. 368 Fog with feeling cold and tired is a classic pairing for an underactive thyroid, which lists all three. 8 Fog with numb or tingling hands and feet points toward low B12. 7 See fatigue, always cold and tingling hands and feet.

Improvement rebounded to premenopausal levels in postmenopause.

Greendale and colleagues, SWAN study, Neurology 2009 4

What tests check for the cause of brain fog?

There is no blood test for brain fog itself, and none for long COVID or depression. 3 The tests a clinician chooses from read the treatable medical causes, picked by your symptoms and history.

TestWhat it looks forWhen it is added
TSH, then free T4Underactive thyroidFog with tiredness, feeling cold, weight gain 8
Vitamin B12, then methylmalonic acidLow B12Older age, metformin, acid reducers, vegetarian diet, tingling 717
Complete blood count, ferritinAnemia, low ironHeavy periods, pale skin, breathlessness 6
HbA1c, fasting glucosePrediabetes, diabetesDiabetes, or episodes of shakiness or confusion on diabetes medicines 19
Sodium (in a metabolic panel)Low blood sodiumWater pills, heart failure, headache with nausea 10
FSH, estradiolMenopauseWomen with changing periods, repeated if needed 15

The panel that checks these

If a clinician tests, TSH, vitamin B12 and a blood count with ferritin usually come first, because they check the commonest treatable medical causes of brain fog. Fasting matters only for a fasting glucose. FSH and estradiol are for women with changing periods and may need repeating, and sodium comes as part of a basic or comprehensive metabolic panel. 1015

  1. FSHPerimenopause and menopause
  2. EstradiolPerimenopause and menopause
  3. TSHUnderactive thyroid
  4. Free T4Underactive thyroid
  5. Vitamin B12Low vitamin B12
  6. Methylmalonic acidLow vitamin B12
  7. FerritinIron deficiency and anemia
  8. HemoglobinIron deficiency and anemia
  9. HbA1cBlood sugar problems
  10. Fasting glucoseBlood sugar problems
  11. SodiumLow blood sodium
Already have results?Add them to LifeFrom and it reads these markers side by side, against each other and over time, in plain language. Free, on the web or in the app.
Add your results Download the app

LifeFrom doesn't sell blood tests. It reads results from any lab and helps you prepare for your own doctor; it doesn't diagnose.

Getting a useful test

  • Describe how it started (suddenly, or over weeks) and what makes it better or worse. 11
  • Bring a two-week note of sleep, mood, alcohol, periods and when the fog is worst.
  • List every medicine and supplement, including over-the-counter allergy and sleep products. 13
  • Mention any COVID-19 infection and whether effort makes things worse. 3
  • One result is a snapshot; see why trends beat single readings.

What may help

Most brain fog eases when the cause is found. These everyday steps help whatever the cause, and none replaces checking the red flags above.

Start with the everyday things
  • Protect sleep: regular times and enough hours; sleep deficiency hurts focus, decisions and memory. 2
  • Move most days; regular physical activity helps thinking, mood and sleep. 16
  • Ask a prescriber or pharmacist whether any medicine, especially a sedating antihistamine or sleep aid, could be adding to the fog. 13
  • If stress, anxiety or low mood is part of it, talking therapies help; call or text 988 if it turns to thoughts of harming yourself. 5
  • After COVID-19, pace activity if effort makes the fog worse. 3
Supplements people look at

Few supplements have good evidence for brain fog, so only B12 is listed, for people who are low. Iron is for a clinician to decide after a test. Supplements support general wellness. They don't treat the causes above, and if a test finds one, treating that comes first. If you take medicines, are pregnant or have a health condition, check with your clinician before starting anything.

Questions to ask your clinician

  • Could any of my medicines be making my thinking foggy?
  • Could this be perimenopause, poor sleep or low mood?
  • Should I have a TSH, B12, blood count, ferritin or blood sugar test?
  • If my B12 is low-normal, is a methylmalonic acid test worth it?
  • Which changes in my thinking would mean I should come back sooner?

This page is general education, not medical advice or a diagnosis. Brain fog has many causes that can overlap, and the clinician who knows your history and examines you is the one to read your results and put them in order.

Free in LifeFromAdd the lab results you already have, and LifeFrom reads each marker next to the others it moves with, in plain language, with the sources it used. Free, on the web or in the app.
Start on the web Download the app
Citations
  1. Cleveland Clinic. Brain fog.
  2. National Heart, Lung, and Blood Institute (NHLBI). Sleep deprivation and deficiency: how sleep affects your health.
  3. Centers for Disease Control and Prevention. Long COVID signs and symptoms.
  4. Greendale GA, Huang MH, Wight RG, et al. Effects of the menopause transition and hormone use on cognitive performance in midlife women. Neurology. 2009;72(21):1850-1857.
  5. MedlinePlus Medical Encyclopedia. Major depression. US National Library of Medicine.
  6. MedlinePlus Medical Encyclopedia. Iron deficiency anemia. US National Library of Medicine.
  7. MedlinePlus Medical Encyclopedia. Anemia due to low vitamin B12 (pernicious anemia). US National Library of Medicine.
  8. American Thyroid Association. Hypothyroidism (underactive).
  9. National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). Low blood glucose (hypoglycemia).
  10. MedlinePlus Medical Encyclopedia. Low blood sodium. US National Library of Medicine.
  11. Centers for Disease Control and Prevention. Signs and symptoms of stroke.
  12. Centers for Disease Control and Prevention. About meningitis.
  13. MedlinePlus Drug Information. Diphenhydramine. US National Library of Medicine.
  14. Gray SL, Anderson ML, Dublin S, et al. Cumulative use of strong anticholinergics and incident dementia: a prospective cohort study. JAMA Intern Med. 2015;175(3):401-407.
  15. MedlinePlus Medical Encyclopedia. Menopause. US National Library of Medicine.
  16. Centers for Disease Control and Prevention. Benefits of physical activity.
  17. Langan RC, Goodbred AJ. Vitamin B12 deficiency: recognition and management. Am Fam Physician. 2017;96(6):384-389.
  18. Centers for Disease Control and Prevention. HEADS UP: signs and symptoms of concussion.
  19. MedlinePlus Medical Encyclopedia. Dementia. US National Library of Medicine.
  20. MedlinePlus Medical Encyclopedia. Weight loss: unintentional. US National Library of Medicine.

Educational context only, not medical advice or a diagnosis. Always discuss your results with a clinician.

Work it out

A pearl held between two fingertipsSleep, cycle and pregnancy · CalculatorOvulation calculatorA calendar estimate of your fertile window.A woman's hand with a fine ring, resting on her pregnant stomachSleep, cycle and pregnancy · CalculatorDue date calculatorFrom your last period, conception or IVF.A gloved hand holding a glucose sensorUnit converters · CalculatorGlucose convertermg/dL to mmol/L, and back.A single drop of blood on a fingertipBlood sugar · CalculatorA1C calculatorAverage glucose to A1C, and back.

Working on this?

SleepSleep deeperSee how LifeFrom helps MindMind performanceSee how LifeFrom helps

Related symptoms

SymptomWhy am I so tired all the time? The common causes, the tests that check them, and which vitamins helpSymptomWhy is my hair falling out? The common causes of hair loss, and the blood tests that check themSymptomWhy can't I sleep? What causes insomnia, and the tests that check the medical causesSymptomNumbness and tingling in hands and feet: the common causes, and the tests that check themSymptomWhy am I always cold? The common causes of feeling cold and cold hands and feet, and the tests that check themSymptomWhat causes dizziness? Lightheaded, spinning or off balance, and the tests that check each

Related guides

LearnNormal blood test values chart: common adult lab rangesLearnIs 5 hours of sleep enough? What the studies found, and the rare people who need lessCompoundsL-tyrosine benefits, side effects and dosage: what the evidence showsConditionsMagnesium deficiency: symptoms, the magnesium blood test and what a low level means

Related markers

PTHParathyroid HormoneSISerum IronTPAThyroid Peroxidase AntibodyTSHThyroid Stimulating HormoneTSIThyroid Stimulating ImmunoglobulinIBCTotal Iron Binding CapacityUIBCUnsaturated Iron Binding Capacity