You might notice it first in small ways — you’ll forget a name, lose your train of thought mid-sentence, or take twice as long to process simple instructions. You may tell yourself you’re just tired, but in MS, this feeling often has a name: brain fog.

Brain fog isn’t vague or metaphorical — it’s a neurological symptom. In fact, cognitive changes are so common in MS that 40–70% of people with MS experience measurable cognitive impairment at some point. PMC+2OUP Academic+2 In relapsing-remitting MS specifically, pooled data show about 32.5% of people meet criteria for impairment across two or more cognitive domains. SpringerLink

Yet, brain fog is often invisible, minimized, or disguised by fatigue, mood, or “just a bad day.” That makes understanding it — and managing it — crucial.

What Is Brain Fog in MS?

In neurological terms, “brain fog” refers to declines or slowdowns across these domains:

For many people living with multiple sclerosis (MS), one of the most frustrating and disorienting symptoms is suddenly losing track of what they were saying — forgetting a word mid-sentence, pausing unexpectedly, or feeling their mind “go blank” during a conversation.

This isn’t classic forgetfulness. It’s a neurologically based symptom caused by how MS disrupts brain function.

MS targets the central nervous system (CNS) — particularly the brain and spinal cord — and damages myelin, the protective sheath surrounding nerve fibers. When myelin is lost, electrical signals between brain regions slow down, misfire, or fail to transmit.

If this damage affects areas involved in language production (e.g., Broca’s area), working memory, or processing speed, the brain may struggle to organize and express thoughts in real time. Even when someone knows what they want to say, the signal may “drop” mid-sentence, mid-thought, or mid-word.

These cognitive-linguistic disruptions are commonly linked to:

Fatigue is one of the most common and disabling symptoms of multiple sclerosis (MS), affecting more than 80% of people with MS at some point during their disease. While physical fatigue is widely recognized, many people experience a less visible but equally debilitating form: cognitive fatigue.

What Is Cognitive Fatigue?

Cognitive fatigue refers to a decline in mental efficiency after sustained cognitive activity. It’s not just about feeling tired — it involves a measurable reduction in attention, processing speed, working memory, and executive function over time.

In MS, this occurs due to demyelination and neuroinflammation in the brain. Damaged myelin slows down or disrupts nerve conduction, forcing the brain to recruit more areas and expend more energy to complete basic cognitive tasks. Functional MRI studies have confirmed that people with MS show increased cortical activation during mental tasks, reflecting this compensatory overwork — which leads to faster mental fatigue.

What It Feels Like