11 Health Problems That Might Be Causing Your Brain Fog

When thinking is unclear, unfocused or slow, you need answers

Kim Painter,

AARP

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Published July 31, 2025

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How’s the weather inside your head? If it’s foggy, you are not alone. You may have what people with a variety of health conditions call brain fog — a common term for thinking problems that can make getting through the day a mental slog.

While there’s no official list of symptoms, people with brain fog often say that “their thinking is not clear, it’s cloudy,” says Lynne Shinto, a naturopathic doctor and professor of neurology at Oregon Health & Science University. “They might say, ‘I have problems concentrating.… I feel like I don’t have clarity of thought, and sometimes I forget things.’ ”

Unlike people with dementia or other, more severe cognitive challenges, they often can do what they need to do in a day, but “their thought processes are just slower,” she says.

Cognitive psychologist Julie Dumas, a professor of psychiatry at the University of Vermont, says brain fog can be like “thinking through mud or molasses.”

Brain fog is not a disease or disorder itself, so it’s important to figure out the underlying cause, Dumas says.

A recent research review published in Trends in Neurosciences found that brain fog has been linked with more than a dozen chronic conditions, and exact symptoms and cognitive test results can vary, depending on the apparent cause. That suggests that there’s no one underlying pathway, the researchers said.

Here are some of the most common possible causes of brain fog:

1. Sleep problems

Not getting enough sleep is a major cause of brain fog. If you have chronic insomnia, which can include problems falling or staying asleep, improving your sleep can clear the fog, says Leslie Swanson, a clinical associate professor of psychiatry at Michigan Medicine.

Another sleep disorder strongly tied to daytime sleepiness and foggy thinking is obstructive sleep apnea, a disorder in which people stop breathing repeatedly during sleep. Many people go undiagnosed and untreated for this condition, Swanson says. “Snoring is a red alert” and a good reason to see a doctor.

2. Perimenopause

Women going through the hormonal shifts of perimenopause, the years before periods end, often report brain fog, with problems like forgetting names or having trouble focusing at work, Swanson says. Studies back them up, she says, finding “small declines” in learning and verbal memory.

Part of the explanation is that perimenopause often comes with sleep disturbances, especially unwanted awakenings during the night, Swanson says. Hot flashes cause some, but not all, of those disturbances, she says.

Hormonal shifts may affect sleep and brain functioning even in women who don’t have a lot of hot flashes, she says. The good news, she says, is that the fog tends to lift after menopause.

3. COVID-19 and other infections

When you get the flu, COVID-19 or any infection, especially with a fever, you can get brain fog, which is likely linked to inflammation, Dumas says. But one reason more people are hearing about brain fog these days is that it’s a common part of long COVID, the symptoms that continue or develop after the initial illness, she says.

Shinto says she has seen some long COVID patients with “intense brain fog, where they have a lot of trouble concentrating [and] intense mental tasks are very draining.”

In addition, many are constantly physically exhausted. That combination, along with other symptoms, can be disabling, making it impossible for some people to manage their usual daily activities, she adds.

4. Inflammatory arthritis

For people with conditions such as rheumatoid arthritis, psoriatic arthritis and arthritis associated with lupus, brain fog is “a pretty universal experience,” says rheumatologist Ashira Blazer, an assistant professor of medicine at the University of Maryland School of Medicine.

These autoimmune disorders involve chronic inflammation, which can directly affect the brain, she says. Pain, loss of sleep, depression, anxiety and other mental health effects of managing a chronic condition can take a toll on clear thinking too, she says. Medications, including steroids used to tamp down inflammation, also can play a role.

5. Fibromyalgia

Fibromyalgia is a chronic condition that causes widespread pain and tenderness, along with fatigue and brain fog, which some patients and doctors call “fibro fog.”

While it isn’t an autoimmune disorder, fibromyalgia is common in people with autoimmune conditions, Blazer says. Whether it’s combined with another condition or not, fibromyalgia may cause the brain to react in ways that might, over time, amplify pain signals and lead to foggy thinking, she says.

Pain medications and poor sleep can also contribute to fibro fog, Blazer says.

6. Thyroid dysfunction

Low levels of thyroid hormone, or hypothyroidism, can slow down your metabolism and lead to brain fog, as well as weight gain, physical exhaustion, depression, hoarseness and dry skin.

The condition is often diagnosed with blood tests and easily treated with thyroid replacement hormones. So if you have unexplained brain fog and haven’t had your thyroid levels tested in more than a year, Shinto says, it’s smart to check them out. “Once you treat the thyroid,” she says, “the brain fog usually goes away.”

7. Nutrient-poor diets

Low levels of vitamin B12, found in poultry, meat, fish, dairy products and fortified foods like cereal, are a common cause of brain fog, especially in older adults, say neurologists Dean and Ayesha Sherzai at the Brain Health and Alzheimer’s Prevention Program at Loma Linda University Health. The two, who teach courses about brain health as The Brain Docs, say you don’t have to be a vegetarian or vegan to be at risk for low B12.

Older adults have more trouble absorbing the nutrient, so they may need more in their diet, Dean Sherzai says.

Diets low in other nutrients, including vitamin D and iron, can also contribute to brain fog, Ayesha Sherzai says.

Dumas and her colleagues are studying whether eating meals high in certain saturated fats — the kind found in many fast foods — might cause a sort of short-term brain fog. But it’s too soon to talk about the results, she says.

8. Dehydration

“As we grow older, our body has a difficult time retaining water, and so even mild levels of dehydration can result in brain fog,” Ayesha Sherzai says. There’s lots of data, she says, showing a link between hydration levels and brain functioning.

9. Medications

Many medications prescribed for mental health conditions, pain and immune system regulation can come with a side effect of brain fog, says Ayesha Sherzai. So can sedating antihistamines. Brain fog from cancer chemotherapy, or chemo brain, is common too, she says.

Blood pressure medications can also be culprits, she says. Problems arise when older adults are treated so aggressively that their blood pressure gets too low, causing wooziness and fuzzy thinking. Adjusting the medication can often help, she says.

One note of caution: If you think a prescription medication is causing brain fog, talk to your doctor. Don’t stop taking your medicine or lower your dose without medical advice.

10. Stress, depression and anxiety

All sorts of mental distress can contribute to brain fog, directly or indirectly, and it’s often difficult to sort out such causes when someone is going through a tough time, Shinto says.

For example, many people under stress or feeling depressed or anxious have trouble sleeping. Others, especially those with depression, may sleep too much, which also contributes to brain fog, she says.

11. Seasonal allergies

Brain fog is “really common with seasonal allergies,” Shinto says.

“If you were fine before the flowers started blooming” and then got foggy, don’t ignore that clue, she says. A simple over-the-counter nonsedating antihistamine might clear your head in more ways than one, she says.

What’s causing your brain fog?

Any change in the way you think and remember deserves attention, because many causes are highly treatable, the experts say.

“A lot of our patients feel very dismissed when they talk about brain fog,” but finding answers is important, Blazer says.

If you aren’t sure if your thinking problems are brain fog or something else, like warning signs of dementia, that needs attention too, Dumas says.

“You know what’s normal for you,” she says. “When it becomes not normal, that should be looked at.”

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Kim Painter is a contributing writer who specializes in health and psychology. She frequently writes for AARP's Staying Sharp and previously worked as a health reporter and columnist at USA Today.