The term “chemo brain” emerged as patients and clinicians began recognizing cognitive changes associated with cancer treatment, including difficulties with memory, attention, concentration, and processing speed². These abilities make up what we call cognition, which is our capacity to think, learn, remember, focus, solve problems, and manage everyday tasks. For many years, chemotherapy was considered the primary cause of these symptoms.

Today, we know the picture is more complex. While chemotherapy can contribute to brain fog, it is not the only factor involved. Brain fog can occur before treatment begins, during treatment, or persist after treatment^{3, 4}. Neuropsychology can help us look more closely at why these cognitive symptoms can occur and why they may affect people differently.

Understanding brain fog through Neuropsychology

Neuropsychology is the field that sits at the intersection between psychology and neurology. It examines how physical changes in the brain influence the way we think (cognition), feel (emotion), and function (behavior) in everyday life. From a neuropsychological perspective, cognitive symptoms can vary in severity and across cognitive domains, with some people reporting subtle difficulties and others experiencing symptoms that interfere with everyday functioning^{4, 5}.  This perspective is particularly helpful when thinking about cancer-related cognitive changes because it encourages us to look beyond the symptom itself and consider what may be contributing to it.

Since the symptoms associated with brain fog can have different contributors, the experience can look different from person to person⁵. Some people notice subtle changes, like occasionally forgetting why they walked into a room or needing more time to find the right word in conversation. Others describe feeling mentally slower, having trouble multitasking, or finding that a task that once felt automatic now requires significantly more effort. Broadly speaking, these experiences can be grouped into common symptoms such as forgetfulness, difficulty concentrating, losing your train of thought, slower processing speed, word-finding difficulties, and challenges with planning or organization. For some individuals these symptoms may be mild, occasional or temporary, while for others they can interfere with work, school, relationships, and everyday responsibilities². Mental fatigue can also accompany these symptoms, but its relationship with brain fog is more complex.

Mental fatigue and brain fog can be difficult to distinguish because they can overlap and affect some of the same cognitive abilities⁶. Feeling mentally exhausted can make it harder to concentrate, process information, and think clearly, and cognitive difficulties can add to the mental effort required to complete everyday tasks⁶. One example of why this distinction matters is memory. When someone is experiencing brain fog or mental fatigue, they may feel like their memory is getting worse or that something is wrong with their memory. However, difficulty remembering something does not necessarily mean that memory is the primary cognitive ability being affected^{5, 7}.  Mental fatigue can make it harder to concentrate and think clearly, while brain fog can affect abilities such as attention, processing speed, and organization,  all of which can make it harder to remember information. 

Recognizing these differences is important because understanding what may be contributing to a difficulty can be a useful first step toward identifying opportunities to address it. A neuropsychological perspective can help us do this by examining how changes in the brain may relate to specific cognitive abilities and how these changes can affect everyday functioning.

How brain fog can happen

Because brain fog describes a range of cognitive symptoms rather than a single condition, there are several ways these symptoms may develop. Researchers have identified a number of factors that may contribute to cancer-related brain fog, including direct effects on the brain, physical and cognitive demands that reduce available mental resources, and psychological and emotional factors^{5, 8, 9, 10}). These factors can be understood through three overlapping pathways (Figure 2).

Figure 2. Mindmap of different contributors to brain fog. AI-generated image.

Pathway 1: Direct Effects on the Brain and Nervous System (CNS involvement, treatment-related effects)

In some cases, the brain and nervous system may be directly affected. This can occur when cancer involves the central nervous system (CNS) or through treatment-related effects on the nervous system. Biological processes such as inflammation and immune system activation may also contribute to cognitive symptoms by affecting the brain and nervous system^{5, 8}. When brain networks involved in cognition are affected, individuals may experience measurable changes in cognitive functioning^{5, 8}. Depending on the regions involved, this can contribute to different cognitive difficulties.

Pathway 2: Reduced Cognitive Resources (fatigue, pain, physical stress)

Not all brain fog results from direct impact to the brain. Thinking requires mental resources, and when those resources are depleted, thinking can become more difficult. Cancer-related fatigue is commonly associated with cognitive difficulties^{5, 6}. Sleep disruption, chronic pain, anemia, medication side effects, and the physical demands of treatment can all reduce the mental resources available for attention and memory^{4, 5, 10}. In these situations, a person may experience greater difficulty using cognitive abilities effectively, even when formal cognitive impairment has not been established⁵. For example, someone experiencing severe fatigue may feel as though their memory is worsening when difficulty focusing on information in the moment makes it harder to remember later.

Pathway 3: Cognitive Overload (overwhelming emotions, mental health)

A third pathway involves cognitive and emotional overload. A cancer diagnosis can introduce uncertainty, stress, medical decision-making, financial concerns, changes in routine, and fears about the future. These experiences can place additional demands on attention and memory. Anxiety, depression, and psychological distress have also been associated with cognitive symptoms in people with cancer^{3, 5, 6}. These factors may make it more difficult to concentrate, process information efficiently, and manage cognitively demanding tasks^{5, 6}. These psychological and emotional factors can meaningfully affect how people experience and function with cognitive symptoms^{3, 5, 6}.

It is important to remember that, in this article, we are referring specifically to brain fog associated with cancer. Because cognitive symptoms can have many possible contributors, persistent, new, or worsening symptoms should be discussed with a healthcare provider rather than automatically attributed to cancer-related brain fog^{4, 5}.

When brain fog can happen

One common misconception about brain fog is that it only develops during or as a result of chemotherapy. In reality, cognitive symptoms can occur before, during, or after cancer treatment^{3-5, 9}.

Before Treatment: Some individuals report cognitive symptoms even before receiving their first treatment^{3, 9}. This may be related to the cancer itself, particularly when cancer directly involves the central nervous system or when biological processes such as inflammation and immune system activation contribute to cognitive symptoms^5,8, as described in Pathway 1: Direct Effects on the Brain and Nervous System. At the same time, receiving a cancer diagnosis can bring stress, uncertainty, and a range of emotional demands that contribute to cognitive overload (Pathway 3: Cognitive Overload).

During Treatment: Brain fog can become more noticeable during active treatment. Chemotherapy, immunotherapy, and radiation therapy may contribute to cognitive symptoms through treatment-related effects on the brain and nervous system (Pathway 1: Direct Effects on the Brain and Nervous System), while fatigue, sleep difficulties, pain, medications, and the physical demands of treatment can reduce the mental resources available for thinking^{4, 5, 8, 10} (Pathway 2: Reduced Cognitive Resources). At the same time, the emotional experience can change as individuals navigate appointments, treatment decisions, side effects, and uncertainty about how treatment is progressing. These emotional demands can contribute to cognitive overload^{3, 5} (Pathway 3: Cognitive Overload). 

After Treatment: For some survivors, cognitive symptoms improve shortly after treatment ends. For others, symptoms may persist over longer periods^{4, 8, 9}. Ongoing physical recovery, pain, medication effects, and reduced stamina can continue to limit the mental resources available for thinking^{4, 5} (Pathway 2: Reduced Cognitive Resources). At the same time, adjusting to life after treatment can bring its own emotional demands, including uncertainty about the future, changes in identity or routine, and concerns about recurrence (Pathway 3: Cognitive Overload). As these factors change over time, the experience of brain fog may change as well.

Because brain fog can arise through multiple pathways, there is rarely a one-size-fits-all solution. Management may involve identifying and addressing the factors most likely contributing to symptoms^{4, 5, 8}. Improving sleep, managing fatigue, staying physically active when possible, and addressing medical contributors with your healthcare provider may help support cognitive functioning^{4, 5, 10, 11}. Many individuals also benefit from practical strategies such as calendars, reminders, written notes, and breaking larger tasks into smaller steps (4). Mental health is equally important: counseling, stress-management strategies, mindfulness-based interventions, and other psychosocial approaches may also help some individuals manage cognitive symptoms or the distress associated with them^{11, 12}. The more informed we are, the better questions we can ask and the more appropriate support we can seek. It is important to discuss persistent cognitive symptoms with your healthcare team. In some cases, a neuropsychological evaluation may help identify specific cognitive strengths and weaknesses and provide recommendations tailored to an individual’s needs¹³.

Brain fog is not a single condition and is not caused exclusively by chemotherapy. It is an umbrella term for a range of cognitive symptoms that can arise from multiple biological, psychological, and treatment-related factors^{5, 8}. Whether those symptoms stem from direct effects on the brain, reduced cognitive resources, or cognitive overload, they can have a meaningful impact on daily life before, during, and after treatment^{4, 5}. Understanding the different pathways that contribute to brain fog can help make the experience feel less confusing and more manageable. Most importantly, it can help patients and survivors recognize that these symptoms are worthy of attention.

Disclaimer: No part of this article should be taken as medical advice. It is solely for educational purposes. Talk to your healthcare providers.

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Natalia Menendez, PhD Student