Brain Fog: Common Causes and Which Tests to Do

The greatest fear of people who notice worsening memory and concentration is dementia. In practice, in young and middle age the great majority of such complaints have reversible or functional causes: sleep, mood, medicines and systemic illness. A few features distinguish them. Brain fog usually has a variable course (better and worse days), worsens after a bad night or under stress, and the person remains independent in daily matters and is usually aware of the difficulties. In neurodegenerative diseases, gradually increasing deficits, loss of orientation or difficulty with everyday activities are most often observed, often noticed by those around before the affected person notices them.

This distinction is a guide, not a diagnosis; it does not replace medical assessment or neuropsychological testing. If the complaints are new, growing month by month and relatives notice changes that the person does not see, it is worth seeing a doctor regardless of age. Later in the article we present a concrete list of alarm symptoms.

If we look for the single most common and best-studied cause of cognitive slowing, it is insufficient sleep. It affects alertness, speed of information processing and working memory, and the effects can be felt as exactly what patients call fog: difficulty sustaining attention, 'zoning out' and errors in tasks that normally cause no trouble.

Significance for the question of fog: alertness and sustained attention suffer most (lapses), and complex reasoning clearly less. This matches the typical picture: the person still 'knows how' to think but keeps 'losing' attention. A caveat is in order: the meta-analysis concerned short-term total sleep deprivation, not chronic moderate sleep restriction or sleep disorders such as apnoea or insomnia, so we transfer the conclusions to everyday life cautiously. We write more broadly about the link between sleep and memory in the entry on sleep and memory.

The second large group of causes is mental health. Complaints of brain fog often accompany depression and anxiety disorders, and chronic stress and burnout produce a similar picture of cognitive fatigue. Importantly, this is not only a 'subjective feeling': in studies using neuropsychological tests the deficits are measurable.

The practical conclusion is twofold. First, difficulty concentrating in a person with low mood, loss of interest or marked anxiety is not 'made up'; it is a documented element of the picture of depression, which may need separate attention in treatment. Second, brain fog without clear mood symptoms does not exclude mild depression; simple screening tools used by a family doctor can help. We write about the link between sleep and mental health separately in the entry on sleep and mental health.

Causes that can be excluded or confirmed with a simple blood test include thyroid disorders (especially overt hypothyroidism, which slows thinking and increases sleepiness), anaemia and iron deficiency, vitamin B12 deficiency, and unrecognised diabetes or large glucose swings. In each of these states brain fog is often one of several symptoms alongside tiredness, pallor, cold hands, hair loss or tingling, so the co-occurrence of symptoms is a diagnostic clue.

One must, however, avoid the simplification 'it is all the thyroid'. In the large TRUST trial of 737 people aged 65 or older with persistent subclinical hypothyroidism (TSH 4.60 to 19.99 mIU/L, free thyroxine within range), levothyroxine, despite lowering TSH, did not change the hypothyroid symptom score or the tiredness score after a year compared with placebo. That does not mean the thyroid cannot cause fog; it means that a mild TSH deviation in an older person is rarely the cause on its own, and treating the 'number' need not improve well-being.

Iron is similarly ambiguous. In a meta-analysis of 18 studies (1408 participants, including 1176 menstruating adults), iron supplementation in randomised trials improved fatigue (d = 0.34), anxiety, physical well-being and short-term memory (d = 0.53), but not attention or depressive symptoms, and the effects disappeared when people with iron deficiency were excluded from the analysis. In other words: the benefit applies to those who truly have a deficiency, including without anaemia, not to everyone who feels 'foggy'. The decision to supplement should therefore rest on the ferritin result and context, and we write about interpreting that result in the entry on ferritin.

Women in the perimenopause often report 'brain fog': problems recalling words and sustaining attention. The mechanism is not fully explained, and the symptom rarely occurs alone: sleep disturbance, mood changes and hot flushes co-occur and themselves worsen cognitive functioning. Data from a descriptive observational study show how hard it is to name a single cause.

This is a cross-sectional study, so it shows associations only, not causation, and the sample was small and self-selected. It does show, however, that subjective fog at this stage of life is closely tangled with psychosocial well-being. For severe or sudden symptoms it is worth discussing with a doctor both the menopause and the exclusion of other causes (thyroid, anaemia, sleep), rather than assuming in advance that hormones alone are to blame. For men we discuss a related topic in the article on testosterone and tiredness.

Brain fog became a widely known term precisely because of long COVID. Persistent cognitive difficulties after viral infections had also been described earlier, however. A meta-analysis of 19 studies and 11,324 patients, assessing neurological symptoms persisting at least 12 weeks after COVID-19, provided concrete figures that help gauge the scale of the phenomenon.

The wide confidence interval for brain fog (from 9% to 55%) shows how strongly results depended on the definition and the assessment method in individual studies, an honest warning against treating a single figure as exact. For the reader, the more important observation is that fog rarely occurs alone: it is accompanied by tiredness and sleep and mood disturbances, so the work-up should consider them together. We write more about symptoms and management in the article on long COVID.

With any new or worsened impairment of thinking it is worth reviewing the list of medicines and substances used. Slowing, sleepiness and trouble concentrating can be caused by sedating or anticholinergic medicines: some older-generation antihistamines, benzodiazepines and sleep aids, certain antidepressants, antiepileptics and drugs used for urinary incontinence, and opioids. Alcohol, even in moderate amounts in the evening, worsens sleep quality and increases fog the next day. Night-time awakenings, noise, sleep apnoea (which we discuss in a separate entry) and caffeine withdrawal act similarly.

The work-up of brain fog should be stepwise. The first step is a history (onset, course, sleep, mood, stress, medicines, infections, head injuries, accompanying symptoms) and a physical examination. The second is basic laboratory tests chosen to fit the symptoms. Only the third step is neuropsychological, imaging or neurological testing, and only when the history or examination indicates such a need.

There is no single test that 'confirms brain fog', and automatically ordering a broad panel without indication increases the risk of borderline results that are hard to interpret and can themselves become a source of worry. For the same reason we do not recommend buying 'fog tests' or many single vitamin tests on your own. It is also worth knowing that in long COVID routine tests are often normal; a practical review for family doctors stresses that the aim of the initial work-up is to exclude coexisting or competing diagnoses, not to 'confirm' the syndrome itself.

Effective management depends on the cause, so there is no single universal 'cure for fog'. In practice the most important things are ordinary ones: regular, sufficient sleep, limiting alcohol, movement, a check of the medicines taken, and treating the causes that have been measured (iron deficiency, hypothyroidism, depression, sleep apnoea). The evidence for popular 'nootropic' supplements in brain fog of unexplained cause is limited and in many cases concerns other populations and indications than marketing assumes; we write about individual substances in separate entries.

Brain fog is a symptom, not a disease, and that is exactly why it is worth treating as a clue, not a verdict. The most common causes are ordinary and reversible: too little sleep, a burdened mood, medicines, iron deficiency, or the energy shortfall that drags on after an infection. The best plan is a patient, orderly one: a symptom diary, a medicine review, basic tests and a quick consultation when alarm signs appear. It is not worth hunting for one miracle supplement or ordering yourself 'all the tests at once'.