Cancer and its treatment often cause fatigue, known as cancer-related fatigue. Between 80 percent to 100 percent of cancer patients report experiencing fatigue, according to the American Cancer Society.

If you haven’t been diagnosed with cancer and you’re experiencing unexplained, persistent tiredness or lack of energy, you may be wondering if your fatigue could be a symptom of cancer.

While fatigue is a common symptom of cancer, cancer rarely causes fatigue alone. Fatigue is often multifactorial, meaning more than one contributing factor may be involved, and none of them may be cancer.

No matter its cause, fatigue is one of the toughest symptoms to deal with. When patients are struggling with fatigue in their daily life, they want to feel better, and they’re looking for someone to help them. As a medical oncologist at Cancer Treatment Centers of America^® (CTCA), I work every day to help patients enjoy a better quality of life while we fight their cancer.

In this article, I’ll cover some common factors that may contribute to cancer-related fatigue. This article examines:

• Ways cancer can cause fatigue
• How cancer treatment may cause fatigue
• Other possible contributors to fatigue during treatment
• How to find help for cancer fatigue

The main focus of this article is on patients who’ve already received a cancer diagnosis. But first, let’s briefly explore fatigue as a symptom of undiagnosed cancer.

If you’re experiencing fatigue during cancer treatment, we may be able to help. We only treat cancer at CTCA^®. Our multidisciplinary team of cancer experts takes an integrative approach to treating cancer and its side effects. Call us or chat online with a member of our team.

Is fatigue a sign of cancer?

Fatigue may develop as a symptom of blood cancers, such as leukemia, lymphoma and multiple myeloma, because these cancers start in the bone marrow, which produces red blood cells that carry oxygen throughout the body.

Fatigue may also be a symptom of undiagnosed metastatic cancer (cancer that has spread to other parts of the body). This is more common in cancers that aren’t typically caught early, such as lung cancer or ovarian cancer.

It’s uncommon for fatigue to be the only symptom of undiagnosed cancer. A more concerning scenario develops when patients experience both fatigue and pain in one area that’s getting progressively worse, along with unexplained weight loss and loss of appetite.

However, these symptoms may also be caused by an infection or another medical condition, such as anemia, depression, thyroid dysfunction, vitamin deficiency or sleep apnea, to name a few possibilities. 

If a patient came to me complaining primarily of fatigue, my first thought—even as an oncologist—probably wouldn’t be cancer.

There may not be a simple answer behind what’s causing your fatigue, and discovering the root cause often takes some digging, along with some trial and error with various therapeutic approaches.

Finding the cause of your fatigue

If you’re experiencing unexplained fatigue that’s concerning you and interfering with your daily activities, start with a visit to your primary care physician to investigate more common reasons for fatigue.

Your physician will probably ask about the level of fatigue you’re experiencing and when it began. He or she should evaluate medications you may be taking for other health conditions because fatigue is a side effect of some medications.

Some common blood tests your doctor may order include:

• A blood chemistry panel to check for kidney disease, liver disease, electrolyte imbalance and signs of cardiovascular disease
• A complete blood count (CBC) to screen for conditions such as anemia, immune deficiency or infection
• Thyroid function tests to evaluate thyroid gland function
• A vitamin D test to determine whether you have vitamin D deficiency

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Other screenings may include a psychological analysis to determine whether you’re depressed or a sleep study to see whether you’re at risk for obstructive sleep apnea.

If those tests are negative, your physician should start looking for another possible underlying cause of fatigue. 

How does cancer cause fatigue?

By definition, cancer is the uncontrolled growth of abnormal cells in the body. That growth requires energy, and cancer absorbs energy that would normally travel to the rest of the body.

It’s uncommon to notice this energy drain, however, unless the cancer has metastasized to organs such as the liver, where it may affect metabolism, or the bone marrow.

When cancer invades the bone marrow, it affects the production of blood cells. White blood cells help fight infection. Red blood cells contain hemoglobin and carry oxygen to and from organs and tissues throughout the body, providing energy and removing waste from cells. Cancer may cause a reduction in the production of red blood cells, resulting in anemia, which causes fatigue.

Inflammation caused by cancer may be another key contributor to cancer-related fatigue. When your body is fighting a disease, it releases chemicals like cytokines to trigger the immune response, but these chemicals may also cause fatigue. 

In patients with advanced cancer, the disease may produce cachexins, which reduce your appetite and may result in cachexia, a serious cancer-related condition that causes substantial weight loss and muscle wasting.

If you’ve been diagnosed with cancer and you’re experiencing fatigue, ask yourself: When did it start? If it started before diagnosis and goes away with treatment, then it’s likely the cancer was causing it. If it starts when you’re in treatment, it may be more difficult to find and treat, especially if a number of factors are contributing to it.

How cancer treatment causes fatigue

Radiation therapy, chemotherapy, surgery, hormone therapy, immunotherapy—these treatments and their common side effects challenge the body, and the body needs energy to recover.

When you start a new treatment, the body produces cytokines to help boost the immune response. When cells (including cancer cells) die, the body works to clean up those dying cells, causing inflammation. It’s a desirable response in this case, but inflammation—both acute and chronic—also causes fatigue.

You can compare this fatigue to how you feel when your body fights the flu or another infection, like COVID-19. While your body’s working hard to fight the infection, you feel tired.

How and when you experience fatigue from cancer treatment may vary according to the treatment you’re receiving and its side effects.

Fatigue with chemotherapy

Take chemotherapy, for example. Chemotherapy works by destroying rapidly dividing cancer cells, but it’s unable to differentiate between cancer cells and normal cells that divide rapidly, such as white blood cells.

Chemotherapy treatment reduces the white blood cell count, and how you feel during the cycle of chemotherapy treatment is usually reflective of how low your white blood cell count is.

During a treatment cycle, you’ll typically feel the peak of your fatigue (usually between days three to 10 of a three-week chemotherapy cycle) when your body has reached the maximum reduction of white blood cell count.

That fatigue would be clearly chemotherapy-related, because it had an identifiable history: The fatigue went up while the white blood cell count was going down, and then it got better—almost back to baseline—by the time you were ready to start the next round of treatment.

Chemotherapy may cause other side effects that can contribute to fatigue. Weight loss and muscle loss may lead to weakness and fatigue. Conversely, some patients gain weight during treatment, increasing the risk of sleep apnea, and consequently, fatigue. And of course, chemotherapy may also affect the production of red blood cells, which may lead to anemia.

Other possible causes of fatigue during cancer treatment

Vitamin D deficiency is common in the general population. Correcting this deficiency in otherwise healthy people has been shown to improve self-reported fatigue. If you’re fatigued due to vitamin D deficiency going into treatment, cancer therapy may be less successful and result in more side effects like peripheral neuropathy (numbness and tingling of hands and feet).

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For example, one study of breast cancer patients being treated with paclitaxel showed that pre-treating vitamin D deficiency reduced the incidence of peripheral neuropathy and also led to fewer treatment disruptions and better treatment outcomes.

Another study showed correcting vitamin D deficiency may ease gastrointestinal mucositis, a painful inflammation and ulceration of the digestive tract, in patients undergoing chemotherapy.

Consider getting your vitamin D levels checked periodically. In my practice, I often check it at least twice a year to get a better understanding of the patient’s vitamin D rhythms from summer to winter. I recommend levels of approximately 50-80 nanograms per milliliter (ng/ml).

When you check your levels is important, though. Levels tend to be higher during the summer, when people are spending more time in sunlight, and lowest at the end of winter. So, if you check it in the middle of the summer and your level is 30, that’s low for the targeted endpoint of 50-80 ng/ml. But if it’s 30 at the end of winter, that means it likely was in the therapeutic range during winter, and your levels may be sufficient.

Vitamin C deficiency is another factor that may affect quality of life, especially in patients undergoing chemotherapy, which may deplete their stores of vitamin C. Keep in mind that unlike most other species, humans are unable on their own to synthesize vitamin C from glucose and require dietary sources to replenish their supply. Some studies show that patients who receive high-dose vitamin C intravenously experienced better quality of life and fewer side effects than those who don’t.

Using intravenous high-dose vitamin C during cancer therapy is considered somewhat controversial by some, however, because many clinical trials haven’t extended beyond phase I and phase II due to lack of industry or government funding support. A systematic review of 23 of these trials concluded that vitamin C was well tolerated in most patients and that the results warrant the need for randomized placebo-controlled trials.

IV administration of high-dose vitamin C isn’t currently covered by medical insurance, though, and it’s expensive to purchase independently because their production is limited. But trying to build vitamin C levels orally, through supplementation and/or nutritional intake, is difficult, since dosing is often limited by risk for gastrointestinal side effects due to its acidity in the stomach.

High-dose vitamin C must be used with caution in some patients. For example, African Americans who have G6PD Deficiency—glucose 6 phosphate dehydrogenase (G6PD) deficiency, a hereditary condition that causes red blood cells to break down under certain conditions—need to be cautious with vitamin C intake.

Depression and anxiety are possible sources of interrelated causes of fatigue during cancer treatment. A cancer diagnosis may provoke or escalate these conditions and may lead to sleep loss and feelings of fatigue. Fatigue is also a potential side effect of antidepressants used to treat depression (as well as medications used to treat pain or peripheral neuropathy). Stress, meanwhile, may lead to high blood pressure, and beta blockers to treat high blood pressure may cause fatigue.

Lack of physical activity increases fatigue. Exercise during cancer treatment may improve physical and mental health while also reducing treatment-related side effects. The National Cancer Institute reports evidence of longer survival in those with breast, colon or prostate cancer who exercise during treatment.

When I advise patients to exercise, though, they often tell me they’re tired from treatment and don’t feel like moving. My response sometimes is, “If you fake having the energy by moving your body even a little bit, it increases your energy levels.” You don’t have to go for a run; you might just go for a guided walk around the house with a loved one or caregiver. Do what you can and build on that.

When I talk to patients who’ve increased their physical activity, they tell me that even though they’re not doing as well as before treatment, the fact that they can do something helps them realize they’re not as weak as they’ve been telling themselves.

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Getting in the habit of a regular exercise program may also help prepare you for survivorship. A meta-analysis examining the effects of exercise in cancer patients suggests it may improve mortality rates and reduce cancer recurrence. So try to include some physical activity in your daily routine.

How to find help for cancer fatigue

It can be hard to pinpoint the cause of cancer-related fatigue, especially if a combination of factors is working together to cause it. There may not be one simple answer. Keep looking for help, even if initial tests for obvious sources of fatigue are negative.

Keep in mind that conventional oncologists aren’t usually trained on all the research that’s been done on how to manage side effects and help improve quality of life. Their focus is on drugs used to treat cancer. But don’t assume your oncologist is the only one capable of managing your side effects, nor the only one who should be following your progress.

Physicians who work in integrative medicine, functional medicine and naturopathy may be more experienced with how to manage those symptoms using relatively non-toxic approaches. These providers may be more likely to know about and use strategies that show the potential (in phase I to phase III studies) to improve patients’ quality of life without causing harmful side effects. However, be cautious with any provider who claims their treatments will cure cancer in lieu of conventional therapy.

Ask whether supportive care services are available where you’re getting your cancer care. Providers, such as registered dietitians who provide nutritional support during treatment, or licensed behavioral health care professionals who focus on the psychosocial needs of patients throughout treatment, may round out your care team in a way that provides you with more comprehensive care. Include your oncologist, but consider also bringing one of these health care professionals on to your cancer care team to help manage side effects.

If you’re not getting the help you’re looking for, consider seeking a second opinion at an oncology center like CTCA. Cancer centers are more likely to offer resources to explore the multifactorial processes that could be affecting fatigue.

How we treat cancer at CTCA

We only treat cancer at CTCA. Our team of multidisciplinary cancer experts takes a personalized, patient-centered approach to treating cancer and its side effects.

In addition to using conventional cancer treatments to attack the cancer itself, we provide evidence-informed supportive care therapies to help patients tolerate treatment and reduce side effects, including:

• Nutritional support, which includes the option of meeting with a registered dietitian who’ll develop a personalized plan for your nutritional needs
• Behavioral health care, which may include working with therapists to help with depression, anxiety and stress, and using techniques such as talk therapy, mindfulness and relaxation techniques
• Access to professionally led support groups for patients and their loved ones, both in-person and online, including our Cancer Fighters community, where you can connect with cancer survivors
• Naturopathic support, which includes consultations with our naturopathic providers who counsel patients on the use of natural, non-toxic techniques to support the healing process

Providers at CTCA work together under one roof, providing convenient access for patients.

Fatigue is a tough challenge to go through. It may be related to your cancer treatments, sleep hygiene, your diet and nutritional deficiencies, psychological factors or other influences. One oncologist, no matter how attentive he or she is, can’t take care of all those issues. It often takes a team of experts to figure it all out.

If you’d like to get a second opinion or talk to someone at CTCA about getting help for fatigue or other cancer-related side effects you’re experiencing, call us or chat online with a member of our team.