The term “brain fog” became very popular during the COVID-19 pandemic, as people struggled with the mind-numbing effects of Covid infection and then, later, with long Covid. But even though it was Covid that got us all talking about brain fog, the term existed for decades before the pandemic.

Yet brain fog is not a recognized psychiatric or medical condition. The medical community has no formal definition for it, nor will you find an entry for it in any diagnostic manual used by psychiatrists. Instead, it’s a loose, nicely descriptive but colloquial term for an array of cognitive symptoms affecting daily life, including lack of focus, forgetfulness, and mental fatigue, that could be ascribed to any number of underlying problems.  Stated most plainly, it’s just as the name suggests: Someone with brain fog is trying to navigate daily life while feeling like their cognition is clouded over, like their brain is obscured by fog or cobwebs. The problems stemming from that sensation can manifest in just about any type of brain function, including memory, language, attention, and executive function (the ability to perform complex tasks).


Because brain fog describes a mental state rather than a discrete medical condition, it has various causes and thus can last anywhere from just a few minutes up to several years. When brain fog is temporary—especially when it’s clearly the result of a temporary circumstance or malady—we call it transient brain fog. When it nags for months or years—especially when the circumstance or malady that caused it has come and gone—we call it long-term brain fog. If, for example, Person A has brain fog while they’re infected with Covid but the brain fog goes away as they recover from the illness, they’ve experienced transient brain fog. Person B might also begin to experience brain fog during Covid, but then, months after they’ve otherwise recovered, the brain fog lingers. Person B has long-term brain fog.


People’s experiences with brain fog vary greatly. For some, it’s a minor annoyance. They find themselves making little careless mistakes at work or have difficulty planning their day. For others, severe, long-term brain fog can be enormously disruptive. Some people have had to leave their jobs or make other drastic changes to their lives because of it.

Brain fog and dementia

The types of cognitive problems some people experience as brain fog are not dissimilar from the symptoms of dementia. That might sound alarming, but post-mortem study of people who’d been severely ill with Covid revealed evidence of micro-clotting in the brain which looks similar to the changes seen in vascular dementia. This suggests that at least Covid-derived brain fog could actually be quite similar to true dementia.

Causes of brain fog

It’s important to remember that, while Covid made “brain fog” a household term, SARS CoV-2 infection is just one of many potential causes. Any infection, including influenza, can cause it, as can hormonal imbalances, depression, lack of sleep, poor diet, stress, lack of exercise, medications including chemotherapy, and, of course, dementia.

Notice that most of the above-named causes are modifiable. If you’re truly concerned about staying mentally sharp, then you’ll want to get at least seven hours of sleep per night, reduce the stress in your life, and eat a mind-healthy diet. You’ll want to seek treatment for depression or hormonal imbalances. You’ll want to protect yourself from infections by getting vaccinated (Although you still might fall ill with Covid, your chances of experiencing brain fog with the infection drop by 13% if you’re fully vaccinated).

Brain fog and Covid

Researchers at Oxford University found that 2.6% of people infected with Covid end up experiencing long-term brain fog. They wondered whether there was something special about Covid, or if the patients would have experienced just as much brain fog if they’d contracted some other infection such as influenza. What they found was that Covid does appear slightly more conducive to long-term brain fog than other common infections. No one is certain as to why, but a few hypotheses have emerged.

The cytokine hypothesis

According to this theory, the memory problems caused by Covid can be linked to elevated levels of a “cytokine” protein, called CCL-11, secreted by immune cells, which causes inflammation in the hippocampus, a part of the brain crucial to memory. When researchers at Stanford University took cerebrospinal fluid samples from Covid-infected mice, they found that the animals’ CCL-11 levels were similar to those of human patients on chemotherapy. And when they injected CCL-11 into non-infected mice, they observed brain changes consistent with cognitive deficit, along with an impaired ability to generate new neurons and create new messaging pathways in the brain.

The microvascular hypothesis

As stated above, autopsies of Covid patients showed evidence of blood clotting in the small blood vessels of the brain. But the same team of researchers, from the University of Lübeck, in Germany, also discovered that a protein called M-PRO, which is associated with SARS CoV-2, damaged the lining of blood vessels all through the body, which makes them more susceptible to clotting.

The hybrid hypothesis

There’s no reason to think that only the cytokine hypothesis or the microvascular hypothesis can be correct. It may be that both things occur as a result of Covid, perhaps even with microvascular damage causing a release of CCL-11, compounding cognitive problems. Researchers are still studying these mechanisms and others to learn just what may be happening in the brain when it’s infected with Covid.

Battling Brain Fog

If you notice a sudden drop-off in memory, an onset of absent-mindedness, a new inability to focus, problems following a storyline on TV, or difficulty staying on task, then you should talk to your doctor, especially if these problems don’t fluctuate or improve. Your doctor will likely perform cognitive testing and then start trying to rule out things like medications, hormone problems and infections.

Of course, you should do all that you can to ensure that your lifestyle is brain-smart, by eating well, exercising and getting enough sleep. If the problems persist without a clear cause, there’s unfortunately very little that can be done. Challenge your brain by staying socially and mentally active and develop strategies for managing daily life while living with brain fog.

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