Countless individuals have described themselves as being lost in a mental haze. Tasks that once seemed simple became daunting. Concentration faltered, memory slipped, and even routine activities began to feel overwhelming. This experience has come to be widely known as “brain fog.”
This clouded mental state is now recognized as one of the hallmark symptoms affecting individuals with long COVID. Estimates suggest that anywhere between 20 to 65 percent of people suffering from long-term COVID-19 effects experience brain fog, a number that varies due to the broad and still-evolving understanding of the condition. Interestingly, while the COVID-19 pandemic may have popularized the term in the public consciousness, it was first introduced within communities managing chronic illnesses.
For years, people coping with conditions like fibromyalgia, lupus, or chronic fatigue syndrome—also referred to as myalgic encephalomyelitis—have voiced similar cognitive difficulties. Even cancer patients undergoing chemotherapy and individuals prescribed certain medications, including opioids, have reported comparable experiences. Furthermore, links between brain fog and psychiatric conditions such as depression and schizophrenia have also been observed.
Despite the diverse range of conditions associated with brain fog, patients tend to describe nearly identical symptoms. Common complaints include challenges with concentration, forgetfulness, mental confusion, and a noticeable slowing in cognitive processing. According to Jacqueline Becker, a neuropsychologist at the Icahn School of Medicine at Mount Sinai, these issues can be profoundly disabling. For many, brain fog becomes one of the most burdensome aspects of their illness. “It can really take over people’s lives,” Becker notes.
For a long time, the root causes behind this foggy mental state remained elusive. However, more recently, progress has been made. Scientific studies are beginning to shed light on the biological underpinnings of brain fog—particularly in the context of long COVID—and how it might eventually be treated.
What Brain Fog Is—and What It Isn’t
The term “brain fog” does not yet have a standardized medical definition. Among clinicians and researchers, there is still debate about whether it should even be used in a diagnostic context. However, a growing consensus suggests that brain fog is best understood not as a condition itself, but as a symptom or collection of symptoms linked to broader underlying health problems.
“Brain fog has become a kind of umbrella term for a variety of neurological symptoms found in conditions like long COVID,” Becker explains.
Its presence across such a wide range of disorders may stem from the fact that it can signal disruptions in several cognitive domains. These might include attention, executive functioning, or the ability to complete tasks effectively. As Avindra Nath from the National Institute of Neurological Disorders and Stroke puts it, “If the brain isn’t functioning properly, people call it brain fog.”
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One key distinction made by experts lies between brain fog and measurable cognitive impairment. While patients often describe issues with memory and focus, standardized clinical tests do not always detect deficits. It is not uncommon for someone with severe symptoms to undergo cognitive evaluations, only for their results to appear within normal limits. This disconnect can be deeply frustrating. “That can be really frustrating for patients,” Becker says.
This lack of measurable evidence has unfortunately led some clinicians to dismiss the condition, attributing it solely to psychological origins. Individuals with chronic illnesses or long COVID, in particular, often report feeling ignored or misunderstood by healthcare professionals. “There is a prevailing perspective that long COVID has a purely psychiatric origin,” says Becker. “And I think it’s important to challenge that.”
Despite the outward similarity in symptoms, scientists increasingly believe that brain fog may stem from multiple causes, each dependent on the underlying condition. This complexity also influences which treatments, if any, may prove effective.
Neuroinflammation: A Common Thread
One of the most promising avenues of research into brain fog has centered around the concept of inflammation within the brain—also referred to as neuroinflammation. It has been suggested by many experts that this may be a shared mechanism driving brain fog across a range of disorders.
According to Becker, neuroinflammation is emerging as a leading theory for what lies behind brain fog. Evidence has shown that infections like COVID-19 can spark an overly aggressive immune response. In some individuals, this reaction does not subside even after the acute phase of the illness has passed.
Several studies have documented how COVID-19 may activate immune cells in the brain over the long term, impairing the growth of neurons and reducing cognitive function. In some cases, the immune system may even begin producing autoantibodies—agents that mistakenly target healthy cells, including those in the brain. Structural changes have also been noted. Imaging studies suggest that COVID-related inflammation can shrink both gray and white matter in the brain, potentially resulting in long-lasting cognitive effects.
It has been hypothesized that viral particles may linger in certain parts of the brain, continually provoking the immune system and fueling ongoing inflammation. Similar inflammatory patterns have been found in conditions like chronic fatigue syndrome and the cognitive symptoms experienced by chemotherapy patients, often termed “chemo brain.”
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A Leaky Blood-Brain Barrier
A major development in early 2024 offered further insight into the biological mechanisms behind brain fog. A study led by neurologist Colin Doherty at Trinity College Dublin examined the brains of individuals experiencing long COVID and identified a potential contributor: a compromised blood-brain barrier.
This barrier serves as a highly selective filter, preventing harmful substances in the bloodstream from reaching the brain. In patients with brain fog, it appeared to be “leaky,” possibly allowing toxins, viruses, and immune system components to penetrate and cause inflammation.
Disruption of the blood-brain barrier has also been reported in autoimmune diseases like lupus and in cases of chronic fatigue. However, some experts advise caution when interpreting these results. The study in question was relatively small, and a separate study conducted a month later failed to replicate the same findings. That discrepancy might be attributed to differences in how participants were selected. In Doherty’s study, only those with measurable cognitive deficits were included.
Beyond Inflammation: Other Theories in Play
Although inflammation remains a dominant theory, other explanations are being explored. Hormonal imbalances have been shown to play a role. For instance, during menopause, falling estrogen levels are believed to shrink certain brain regions, potentially contributing to mental fogginess.
Similar effects have been noted in those with thyroid disorders. In cases of hypothyroidism, a lack of thyroid hormone may reduce the size of the hippocampus, a brain region critical for memory.
In patients who have experienced traumatic brain injuries, symptoms similar to brain fog have been associated with decreased levels of growth hormone.
Another area of interest involves the gut-brain connection. Research has increasingly linked gut microbiome imbalances to cognitive issues. A small study published in late 2024 found that over half of individuals with gastrointestinal disorders, such as inflammatory bowel disease, reported brain fog. Some scientists now believe that gut dysbiosis—an imbalance in the microbial ecosystem—may also play a role in long COVID.
Despite these possibilities, many aspects of brain fog remain poorly understood. One of the key barriers to progress has been the lack of large, high-quality studies on the topic. According to Denno, many existing studies differ significantly in their methodologies and definitions, making comparisons difficult. For instance, the relationship between chronic pain and brain fog continues to be obscured by inconsistent findings.
For now, Denno suggests that cognitive symptoms should be studied within the context of each specific illness, using clearer criteria and more standardized research tools.
Managing and Diagnosing Brain Fog
Though no single cause of brain fog has been identified, several strategies may help in managing its effects. Becker advises that individuals first examine their lifestyle habits. Regular exercise, balanced nutrition, and sufficient sleep can all contribute to improved cognitive health.
When symptoms are severe or persist for several weeks, it is recommended that individuals consult a medical professional. Conditions such as sleep apnea, vitamin B deficiencies, or hormonal imbalances can sometimes be identified and addressed. Inflammation markers and indicators of neurodegenerative disease can also be assessed during evaluation.
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For those experiencing measurable cognitive impairment, cognitive rehabilitation therapy may offer relief. This approach involves mental exercises targeted at the areas most affected. Becker emphasizes that this type of therapy is non-invasive and can yield substantial benefits. Nath describes it as “exercise for the brain,” designed to strengthen weakened cognitive pathways.
Pharmacological treatments are also being explored. Some patients experiencing post-chemotherapy brain fog have reported improvements when treated with ADHD medications. Other studies have examined the use of antihistamines or famotidine—an antacid known for its anti-inflammatory properties—with some positive early findings.
Experimental therapies are also being investigated. Nath and his colleagues are studying intravenous immunoglobulin, commonly used to treat autoimmune diseases, in the hopes that it may reduce excessive immune activity in long COVID patients. There is also interest in testing checkpoint inhibitors—drugs typically used in cancer treatment—to determine whether they might help modulate immune responses and reduce brain fog symptoms.
A Call for Continued Research
Despite these advances, much remains to be discovered. Experts agree that the medical community still lacks a comprehensive understanding of what brain fog truly is and how best to treat it. As Doherty remarks, “There’s this idea in medicine that if we can’t break a symptom down into a Latinized medical term then we’ve failed.” But perhaps, he suggests, it’s simply a signal to keep looking deeper.
For now, brain fog remains an enigmatic and deeply disruptive symptom, one that continues to challenge patients and researchers alike. But as scientific tools and understanding evolve, the hope is that the fog may one day lift—both metaphorically and literally—for millions who suffer from it.
