What a clinician looks for when you say ‘brain fog’

It’s mid-afternoon and you’ve read the same email three times. Halfway through a sentence, an ordinary word vanishes. You walk into the kitchen and can’t remember what you came for. Type “brain fog” into a search bar and the results split two ways: products promising to clear it, and forum threads insisting doctors will brush it off.

Both responses skip a step. Brain fog is a phrase patients use, not a diagnosis clinicians make, and it covers a surprisingly wide range of experiences. That is the reason a careful clinician answers it with questions rather than a prescription or a shrug, not a reason to dismiss it.

One phrase, many different complaints

In October 2021, researchers at the University of Edinburgh collected a week of Reddit posts that mentioned brain fog. Of 1,663 posts, 717 met their criteria, and 141 described the experience in the first person. Their paper in the Journal of Neurology, Neurosurgery and Psychiatry sorted those descriptions, and they fall into a few families:

• Memory and attention. Forgetfulness was the most common description, in 51 posts, and trouble concentrating came next, in 43.
• Feeling. Thirty-four described feeling detached or unreal, which clinicians call dissociation. Another 10 described fuzziness or a sense of pressure in the head.
• Speed and effort. Twenty-six described thinking that felt slow or took extra effort, and 9 described fatigue.
• Language. Twenty-two described difficulty communicating.

The posters’ explanations varied just as widely. About half the posts sat in forums about illnesses, and 134 were in forums about drug use or stopping a drug. Among posters who named a cause, long COVID was the most common, yet it accounted for only 10%.

Laura McWhirter, the first author of that paper, followed up in 2025 with a review for neurologists in Practical Neurology. Doctors may think they know what a patient means by brain fog, she wrote, but “at least some of the time we are likely to be getting it wrong.” The review suggests lines of questioning to pin the experience down.

That’s the practical point. Losing words mid-sentence and feeling like you’re watching yourself from a distance lead to different questions, and possibly to different causes. It’s also why a product sold to clear brain fog is aimed at a category rather than a cause; claims made for brain-health supplements get their own scrutiny in a later part of this series.

What happened when 112,964 people were tested

The best measured data come from England. Researchers running the REACT study, a large community survey that tracked COVID infections, invited 800,000 adults to complete eight online cognitive tasks between August and December 2022. Of 141,583 people who started, 112,964 finished all eight, and the team published its analysis in The New England Journal of Medicine in 2024.

Compared with people who had no confirmed infection, those whose symptoms had cleared scored about 0.2 standard deviations lower on a combined score. The authors classed that as small and translated it themselves: roughly 3 points on a typical IQ scale. People whose symptoms had lasted 12 weeks or more and still hadn’t resolved scored about 0.4 standard deviations lower. People who had been in intensive care showed the largest gap, 0.63 standard deviations in a matched comparison, which the authors put at about 9 IQ points. Memory, reasoning and planning tasks showed the biggest differences.

The gaps were smaller for people infected later in the pandemic, and people with two or more vaccine doses had a small advantage. The study was observational and had no test results from before anyone was infected, so it can’t show that the infection caused the difference, and the authors say so. In this library’s evidence vocabulary, the deficit after a resolved infection is measured, and its own authors call it small.

Then comes the finding that matters most for anyone using the phrase. Everyone was asked whether they’d had poor memory or brain fog in the previous two weeks. Among people who’d had COVID, those who said yes scored somewhat lower on some tasks, a weak link. Among 46,261 people in the no-COVID group, the link was negligible: scores looked about the same whether or not people reported brain fog.

That cuts in two directions. Feeling foggy and scoring lower on a test are not the same thing, so a test battery can come back unremarkable while you still feel impaired. But the researchers did not conclude that brain fog is imaginary. They wrote that “although such symptoms are imprecise, they can reflect objectively measurable deficits.” The word alone can’t tell anyone, you included, which situation you’re in. The rest of the evaluation exists to sort that out.

Brain fog also counts toward the research definition of long COVID developed by the NIH-funded RECOVER study, which in 2023 scored symptoms reported by 9,764 adults; a 2024 update, published in 2025, kept brain fog in the index. Long COVID as a condition, with its own definitions and evidence, is covered in a separate entry on lingering post-infection symptoms.

The first questions are about sleep, mood, medicines and alcohol

There is no clinical guideline for brain fog as such. Its nearest relative is fatigue, which overlaps with it, and a 2023 review in American Family Physician by Kelly M. Latimer, Althea Gunther and Michael Kopec sets out how a primary care workup for fatigue runs. It starts with the history. The clinician asks about sleep and whether it’s restful, and about thinking: trouble with driving, reading, timed tasks, conversation or multitasking, compared with how things used to be. The review recommends validated questionnaires to screen for sleep, mood and substance-use problems, and a medication review to make sure the problem isn’t a side effect.

Each of those threads points to something a clinician can check.

Sleep. Someone with sleep apnea stops breathing again and again overnight, often without knowing it; a bed partner tends to hear the snoring and gasping first. The daytime result, according to the National Heart, Lung, and Blood Institute, can be trouble learning, focusing and reacting, and in women the picture leans toward fatigue, headaches and insomnia. What sleep apnea means for the brain over decades is taken up in our article on sleep and long-term brain health.

Mood. Some of depression’s symptoms are cognitive ones. The National Institute of Mental Health’s list includes feeling slowed down and struggling to concentrate, remember or decide, and the institute adds that thyroid disorders and some other medical conditions can mimic depression, one reason a clinician may check both. If low mood comes with thoughts of suicide, call or text the 988 Suicide and Crisis Lifeline at 988. In a life-threatening situation, call 911.

Medicines. The most developed guidance here covers adults 65 and older. The American Geriatrics Society’s 2023 Beers Criteria, a widely used U.S. list of potentially inappropriate medications for older adults, expanded its warning on anticholinergic drugs (medicines that block a nerve-signaling chemical called acetylcholine) to recognize the added risk of taking several at once. It flags the use of three or more drugs that act on the central nervous system from specific categories, a group that now includes skeletal muscle relaxants, and it added opioids to the drugs that can worsen delirium. The same panel stresses that its “avoid” label is not an absolute ban and that stopping a medicine safely takes a plan, naming tapering and withdrawal symptoms among the challenges. Whether a medicine is contributing to fog is a decision for whoever prescribed it, made with the full list in front of them. Our article on having one pharmacist review everything you take explains what the list does and doesn’t mean.

Alcohol and other substances. Recall that nearly one in five of the Edinburgh posts sat in forums about drug use or stopping a drug. Alcohol, recreational drugs and any recent change to prescribed medicines all belong in the history; the research on drinking and cognition sits in our coverage of nutrition and the brain.

Tests follow the history, not the other way around

People often hope a large blood panel will find the answer. The AAFP review is blunt about why that usually disappoints. Lab testing without a specific reason, it says, is not high-yield and may change treatment in only about 5% of patients with fatigue. In a study it cites of young adults seen in primary care for new fatigue, 27% received a diagnosis that could explain it, most often anemia, vitamin B12 deficiency, an infection, pregnancy or a psychiatric condition.

Its first-listed recommendation, rated as consensus and expert opinion, is to check for physiologic fatigue, an imbalance between the activities that use energy and those that restore it, before investigating other causes. The same review notes that brain fog can count toward the diagnostic criteria for myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS), in which a crash after ordinary activity, called post-exertional malaise, is the most important feature.

Which targeted tests make sense depends on the history. Two examples show how that works.

Thyroid. Tiredness and forgetfulness both appear on the American Thyroid Association’s list of underactive-thyroid symptoms, alongside a caution that plenty of other problems produce them too. Diagnosis rests on two blood tests, TSH and free T4. How a hormone evaluation should run, and why one abnormal result isn’t the end of it, gets full treatment in our companion article on hormone testing and claims.

Vitamin B12. The NIH Office of Dietary Supplements lists several groups at higher risk of deficiency: older adults, particularly those with a stomach condition called atrophic gastritis; people with pernicious anemia; people who have had stomach or intestinal surgery, including weight-loss procedures; and vegans. Acid-reducing drugs, meaning proton pump inhibitors and H2 blockers, can impair absorption of B12 from food, and metformin may lower B12 levels. The office notes that the neurological effects of deficiency can appear even without anemia. B12 deficiency is examined in more depth alongside the diet and cognition trials.

Fog that comes with a known cause, and often an end

Some brain fog arrives with a recognizable life event, and the research suggests several of these episodes are temporary.

Perimenopause. The Study of Women’s Health Across the Nation followed 2,362 women for four years and tested them repeatedly. People usually score a little higher each time they retake a test, because they learn it. During perimenopause that improvement stalled: women in late perimenopause didn’t gain on a processing-speed test, and women in early and late perimenopause didn’t gain on verbal memory. After menopause, the learning effect returned to premenopausal levels, and the authors, writing in Neurology in 2009, suggested the difficulties may be limited in time. The evidence is observational, but it tracked the same women over years, which is stronger than a one-time snapshot.

Pregnancy. A 2018 meta-analysis in the Medical Journal of Australia pooled 20 studies covering 709 pregnant and 521 non-pregnant women. Overall cognitive scores were lower during pregnancy by a moderate margin, about half a standard deviation, and the gap was clearest in the third trimester. The authors said the effect on everyday functioning still needs study.

Cancer treatment. The National Cancer Institute uses the term chemo brain for thinking and memory problems in people with cancer, and names chemotherapy, some types of radiation to the brain and immunotherapy among the causes. It also lists contributors that can be treated in their own right: poor nutrition, anxiety, depression, fatigue and insomnia. NCI advises telling the care team about these problems and asking about specialists such as a neuropsychologist or an occupational therapist. Its pages give no figure for how many patients are affected.

After Lyme disease. Here the documented harm comes from a treatment people seek out. The CDC says extended antibiotic treatment is no better than placebo for lingering symptoms and has been linked to serious, even deadly, complications such as sepsis. CDC uses the term post-treatment Lyme disease syndrome for fatigue, body aches, difficulty thinking and memory problems that persist after treatment, and discourages the label chronic Lyme disease because it implies an ongoing infection. It says these symptoms are 5% to 10% more common among people who have had Lyme disease than among people who haven’t, which is a difference between groups, not the share of patients affected. Patients usually get better over time without more antibiotics, though feeling fully well can take many months.

When it isn’t fog: sudden confusion is an emergency

Everything above assumes fog that builds slowly or lingers. Confusion that arrives suddenly is a different category.

The CDC lists sudden confusion, trouble speaking or difficulty understanding speech among the warning signs of stroke. Its B.E. F.A.S.T. check runs through balance, eyes, face, arms and speech, with T standing for time to call 911. Call 911 rather than driving, and keep track of the clock: CDC says the most effective stroke treatments depend on diagnosis within three hours of the first symptom. The full list of 911 symptoms, and why people hesitate to call, is in our emergency-symptoms guide.

Delirium is the other sudden form. MedlinePlus, from the National Library of Medicine, describes it as a state of being confused, disoriented and unable to think or remember clearly. It usually starts over a few hours or days, the symptoms come and go, and alertness often fades toward night. Triggers include infections such as urinary tract infections and pneumonia, sedatives and opioids, withdrawal from alcohol or medicines, dehydration and surgery. Older adults, people with dementia and people in hospital are among those at higher risk. MedlinePlus notes that treating the cause often leads to full recovery. A sudden change like that in a parent or partner calls for a same-day call to their clinician, or 911 if they can’t be woken, not a note for next month’s appointment.

A fog log to bring to the appointment

Because brain fog means different things to different people, the most useful thing you can bring is a precise description of yours. Start a short daily note now and bring whatever you have; there’s no need to put off booking the visit until the log is done. The log is for fog that comes and goes or builds slowly, not for anything urgent. If the confusion came on suddenly, skip the log and get help now.

Each day, jot down:

• Which kind of fog. Lost words, lost thread, slow thinking, forgetting, a detached or unreal feeling, or plain tiredness. Your own words are fine; vague ones aren’t.
• When it starts and lifts. Clock times beat “afternoons.”
• The night before. Hours slept, whether anyone heard you snore or gasp, and whether you dozed off during the day.
• Everything you took. Prescription and over-the-counter medicines with the time of each dose, including sleep aids and allergy pills, plus alcohol or other substances. Flag any new medicine or dose change in the past three months.
• Your context. Day of your menstrual cycle or date of your last period, pregnancy, dates of cancer treatment, a recent infection or a tick bite.
• Mood. A one-line rating is enough.

Once a week, add one concrete example of what the fog cost you: a missed highway exit, a bill paid late, a meeting you couldn’t follow. A real-world example tells a clinician more than any adjective.

At the visit, one question puts the targeted-testing logic to work: given this pattern, which tests would change what we do next? If memory trouble seems to be building slowly over years rather than weeks, say so plainly, because that calls for a different kind of assessment, which our coverage of memory changes takes up. And if you leave feeling dismissed, our article on what to say when a doctor isn’t taking you seriously offers wording that helps.

What a good evaluation can and can’t promise

A careful workup won’t always end with one named cause. Brain fog can have more than one contributor at once, a short night stacked on a new medicine stacked on a hard month, and sometimes the result is a list of small fixes rather than a diagnosis.

What it can do is check the common contributors that respond to treatment, such as sleep apnea, depression, medication effects, an underactive thyroid and low B12. It can put a time frame on the fog that comes with perimenopause, pregnancy, cancer treatment or a recent infection. And it can rule out the emergencies. If the evaluation finds nothing and the fog persists or worsens, that’s a reason to go back, not proof that nothing is wrong.

The single most useful thing you can do before any of that is describe your fog precisely: what kind, when, and alongside what. The phrase gets you in the door. The details are what a clinician can work with.

Sources

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