I treat brain fog. Here are 6 surprising causes many people overlook. | Expert Opinion

Q: A lot of people are talking about brain fog, and the symptoms sound exactly like me. How do I know if I have it, and is there a treatment?

A: You walk decisively into the living room and pause just past the doorway. Why are you here? What was it you came to look for again?

Maybe you’re chatting with a friend and there’s a phrase at the tip of your tongue — you were about to say something clever, but now you can’t find the right words. You feel embarrassed and annoyed with yourself.

Or perhaps it’s not just one precise moment in your day — maybe the entire world feels enveloped in a haze you’re struggling to make sense of.

That’s how a lot of my patients describe brain fog, a term that’s not an official diagnosis but is showing up in the medical literature more and more. Much of the recent discourse has focused on long COVID, but brain fog has become an important part of the conversation in many conditions, including perimenopause and hypothyroidism. It’s become enough of a buzzword that I’ve also heard from people wondering whether it’s even “a thing.”

“Brain fog is real,” said Adam Hampshire, a brain fog researcher and a professor of cognitive and computational neuroscience at King’s College London. “It’s affecting people’s decisions, and it’s affecting their life. It’s really important not to be dismissive where that’s the case.”

For me and most clinicians treating it, brain fog becomes a condition to investigate when it starts to interfere with your daily activities. On the extreme end, it can hinder the ability to hold down a job or care for family.

But brain fog itself isn’t the diagnosis — it’s a symptom, Hampshire said. It should be viewed as a starting point to investigate the underlying cause. In some cases, people perform just fine on cognitive tests, but the tests feel harder to do. That’s part of why brain fog research is complicated, and people can feel dismissed if their cognitive tests come back “normal.” Brain fog probably isn’t one cohesive condition but a shared pattern of symptoms with many roots.

Brain fog is an underappreciated symptom associated with common conditions like depression, anxiety, low thyroid function, and even early dementia. These are often the first diagnoses I consider — they can be assessed, and, if present, treated accordingly.

Here are six other surprising conditions associated with brain fog — and what the data shows may help.

It turns out brain fog, in many cases, traces back to the gut. At least half of people with common GI disorders experience it. That’s why neurogastroenterologists like myself who study the gut-brain connection have come to focus so closely on what I call “gut fog.”

In those with celiac disease, the overwhelming majority — upward of 80% — experience brain fog in addition to such classic GI symptoms as bloating and diarrhea. For about one-quarter, brain fog is the only symptom. Gluten triggers small bowel inflammation that’s thought to spill over into the rest of the body, including the brain. In a small study of newly diagnosed patients with celiac disease, cognitive testing improved over a year on a gluten-free diet, and this correlated closely with how well the small bowel had healed.

That close relationship between food and brain function isn’t limited to celiac disease: Ultra-processed foods are linked to lower attention scores and higher dementia risk even in the general population, probably through inflammatory pathways.

This is why I advise most of my patients to eat an anti-inflammatory, Mediterranean-style diet that focuses on whole foods and fiber.

In 2024, a group of researchers from Ireland published a study of long COVID patients in Nature Neuroscience involving persistent changes to the blood-brain barrier, the rugged membrane that regulates what can pass from bloodstream to the brain.

The clue had come from an unexpected place: a study that found rugby players and martial arts fighters with memory problems that lingered for months after a concussion, even with normal head CTs, who turned out to have disrupted blood-brain barriers. So, in 2020, when patients who had recovered from COVID started describing lingering “brain fog,” it sounded familiar.

Using a special contrast dye injected into the bloodstream, the scientists compared a group of long COVID patients who reported brain fog with a group of long COVID patients who didn’t. In those without brain fog, the dye stayed where it should — outside the brain. But in the patients with brain fog? It leaked through, and their brain scans lit up.

Other studies on brain fog in long COVID patients have shown changes to the electrical waves in the brain and signs of poor blood flow in certain areas.

So what actually helps here? The honest answer: We’re still figuring that out, but there have been some promising studies. A randomized trial published in 2026 found that 10 weeks of cognitive rehabilitation, one-on-one video sessions that help patients build concrete strategies to meet their own goals, led to improvement in brain fog symptoms that lasted at least six months.

Brain fog is one of the most common complaints of the menopause transition. Up to 60% of women experience some form of it — often trouble with finding words or remembering names or numbers. Data from the Study of Women’s Health Across the Nation suggests that for most women, these changes are mild and do not persist after perimenopause.

The leading theory is related to the role of estrogen in the brain, specifically the regions governing memory and executive function. Women who experience hot flashes tend to perform worse on memory testing.

Most high-quality randomized trials have not found a cognitive benefit from hormone replacement therapy for people going through natural menopause — even when hot flashes and overall quality of life improve.

In general, the same important principles to protect the aging brain still apply: regular exercise, with resistance training being especially important for women during menopause; a Mediterranean-style diet; staying socially engaged; and minimizing alcohol intake.

People often equate sleep apnea with snoring, but that’s not necessarily the case: Up to 40% of women with sleep apnea don’t snore, don’t have nighttime choking episodes, and don’t have witnessed episodes during which they stop breathing.

Brain fog, however, is a common complaint, and objective cognitive impairment affects over a third of people with sleep apnea. A four-year study of over 1,100 adults found that people with new or existing sleep apnea showed faster declines in attention, visual processing, and memory, with measurable changes in the brain’s white matter, compared with adults without sleep apnea.

It’s estimated that 75% of women with sleep apnea remain undiagnosed. Early recognition is key because treatment with CPAP therapy can make a big difference.

People who are at risk of vitamin B12 deficiency, such as adults older than 65, those who have had certain gastrointestinal surgeries, and vegans, may benefit from an evaluation if they experience brain fog.

It’s worth noting: A study from 1988 in the New England Journal of Medicine found that 28% of patients with neuropsychiatric symptoms from a B12 deficiency had normal blood levels.

But if you have risk factors for a deficiency, a different set of blood tests can help get to the bottom of it.

This absolutely doesn’t mean we should all be rushing to get extra B12. If you don’t have a deficiency on a blood test and have no specific risk factors, further testing may not be warranted, and vitamin B12 supplementation can be a waste of money.

Cognitive difficulties are so prevalent in fibromyalgia, a chronic pain condition, that patients have their own name for it: “fibro fog.”

The prevailing hypothesis is that managing constant pain consumes brain bandwidth and resources that would otherwise go toward memory and attention. And because many people with chronic pain sleep poorly, their symptoms compound.

Besides addressing sleep issues and other pain management strategies, moderate aerobic exercise has been shown to improve both cognitive performance and overall fibromyalgia symptoms.

There are many other conditions linked to brain fog, including multiple sclerosis and fatigue.

It’s important to discuss brain fog early with a physician in part because there are so many potential underlying causes — several of which we can treat effectively.

Trisha Pasricha is a physician and scientist at Beth Israel Deaconess Medical Center and an assistant professor of medicine at Harvard Medical School. She is the author of “You’ve Been Pooping All Wrong.”