23 Feb 2022

What causes brain fog?

author Ashley Owen
Do you have trouble keeping your mind on a task, or not feeling quite as sharp as usual? People who experience issues like this often say they’re suffering from ‘brain fog’, but what exactly does the term mean? We look at what causes brain fog and how you can return to feeling your best.

What is ‘brain fog’?

Brain fog is not technically a medical diagnosis. Rather, it's a phrase that is popularly used to describe feeling a little bit ‘out of it’, having trouble concentrating, or struggling with tasks you normally find easy. 

It’s a nebulous term without any fixed symptoms, making it tricky to pin down. Similarly, it is associated with lots of other health problems. For instance, many people who have experienced ‘long covid’ report brain fog as one of their symptoms. (We’ll talk more about this below.)

People may also experience it slightly differently, as brain fog is both subjective and personal. For instance, you may find a task - say, managing a spreadsheet - easy when you’re in full health yet struggle if you are experiencing brain fog. Yet your friend or colleague might never find working with spreadsheets easy! This means measuring brain fog objectively can be quite tricky. 

So, how do you know if you’re experiencing brain fog?

Symptoms of brain fog

Most of us have experienced brain fog at some point in our lives. Imagine how it feels when you get up early and haven’t had your morning coffee yet; you’d struggle to do a cryptic crossword, even though you may race through one at 4pm!

Alternatively, perhaps there’s been an occasion when you’ve had a lot on your mind. Perhaps you’ve been at work while one of your kids was mildly ill. Even though you may not be actively worrying about them, you suddenly find that it’s 3pm and you’ve forgotten a meeting at 2pm.

Many people also commonly report brain fog after a bout of illness. Much as feeling unwell leaves us feeling physically tired for a while afterwards, we can also experience mental fatigue. You might normally love to curl up with a book at the end of a long day, but find yourself unable to keep your mind from wandering. 


How brain fog manifests itself varies from person to person, but we all know how we perform mentally on a normal, healthy day. So, if you’re feeling as though you’re struggling to keep up with your own cognitive standards, you’re probably experiencing brain fog. 

What causes brain fog?

As we’ve hinted above, brain fog can arise for lots of different reasons. Some are physical while others are psychological. 

Physical reasons for brain fog include the effects of medications or other substances. This is down to the fact that they contain chemicals which have a direct impact on the way our brains work. 

For instance, alcoholic drinks are notorious for lowing our ability to process information and make decisions.

However, medical treatments such as chemotherapy can also result in brain fog. This is known colloquially as ‘chemo brain’. This may pass fairly quickly, or become a longer-term issue. 

Pregnancy and menopause are also key times when women often experience brain fog. It’s thought that this is because of the change in hormones brought about during these periods. It’s common for new mums to joke about experiencing ‘baby brain’!

Psychological reasons for brain fog include stress, anxiety or a lack of sleep.


Recently, a new pattern has emerged wherein people who have become ill with COVID-19 and subsequently recovered experience unpleasant long-term symptoms. This has become known as ‘long covid’, and brain fog is a commonly-cited symptom. 


Because COVID-19 is such a new illness, there is still much we don’t know about how it affects our bodies. Similarly, little is known about ‘long covid’, when a patient experiences feelings of tiredness, mental fatigue or other symptoms for a long time after the original illness has vanished. 

However, from the information available, some scientists have begun to look into the link between brain fog and COVID-19. 

How COVID causes brain fog

Some have suggested that this might be down to increased microthrombi (very small blood clots) in the brain, or micro-structural changes to areas such as the hippocampus.(1)

Changes such as these appear to be linked with mild cognitive impairment.

Meanwhile, others have postulated that it may be down to a particular part of our immune response when trying to fight off COVID. This is called a cytokine storm.

Cytokines are ‘messenger’ proteins which play a part in the normal functioning of your immune system. However, in a severe immune response, too many can be released into your bloodstream. This can have harmful effects on your organs, including your brain.(2)

Having COVID in itself, or the treatments used to help you beat it, may influence parts of your brain which play a role in cognitive function. Further, the stress and upset of having the illness may also play a role.(2)

It also appears that COVID-19 related brain fog may be more common in women than men, and more likely in patients who were admitted to intensive care.(2)

As you can see, there are lots of factors at play here and there is much we still don’t know. 

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