They tell you that your hemoglobin is low, and you don’t know if it’s because of the tumor, the chemo, or something else. This is a very common concern. Here we explain the most common causes, how your care team investigates them, and what options are available. If fatigue is your main symptom, you’ll find more information in our guide on asthenia (#30). If you have a fever during chemo, check out the guide on febrile neutropenia (#43).
What Is Anemia and How Do You Know If You Have It?
Anemia is a low level of hemoglobin, a protein in red blood cells that carries oxygen throughout the body. Chemotherapy treatments and the disease itself can cause anemia.
It’s not uncommon. A large European survey of more than 15,000 cancer patients found anemia in 39.3% at the onset of the disease and in 67% after six months. These figures cover various types of tumors, not just colon cancer.
The most common symptoms are:
- Extreme fatigue.
- Heart palpitations or a rapid heartbeat.
- Shortness of breath during physical exertion.
- Dizziness or lightheadedness.
- Paleness of the skin, nails, mouth, or gums.
- Swelling of the hands or feet.
This list matches that of the American Cancer Society. Note: Anemia can be mild or severe, and heart or lung problems can worsen your symptoms.
Why it occurs: There is no single cause
In cancer, anemia is usually multifactorial. In other words, several causes combine. The main ones are as follows:
- Bleeding from the tumor. A colon tumor can cause iron-deficiency anemia due to chronic bleeding. Sometimes the blood isn’t visible to the naked eye. A study of patients with colon cancer estimated that about 20% experience gastrointestinal bleeding, which likely causes iron-deficiency anemia. That is why, in many cases, anemia is what leads to the diagnosis.
- Chemotherapy. Some drugs interfere with the bone marrow’s ability to produce red blood cells. A review by the SEOM (Spanish Society of Medical Oncology) lists chemotherapy-induced bone marrow toxicity among the treatment-related causes. This becomes more noticeable the more cycles you undergo, and it depends on the treatment regimen and its intensity.
- Chronic inflammation (“anemia of chronic disease”). This involves a shortened lifespan of red blood cells, reduced production in the bone marrow, and inadequate mobilization of iron from stores.
- “Functional” iron deficiency. Iron is present in the body but is not available in sufficient quantities to produce hemoglobin.
- Other causes: vitamin deficiencies, poor diet, kidney problems, or destruction of red blood cells. Among the causes cited by the Ministry of Health are nutritional deficiency, bleeding, and chemotherapy.
Another possible cause is medication. Long-term use of acetylsalicylic acid, ibuprofen, or other blood-thinning medications can cause gastrointestinal bleeding. If you’re taking any of these, don’t stop on your own. Discuss it with your oncologist or primary care physician, who will determine whether you should continue taking them.
How It Is Diagnosed
Hemoglobin levels are measured with a simple blood test. But the important thing is to find out why it’s low, because treatment depends on that. The SEOM guidelines indicate that the basic tests for evaluating anemia are simple and should be part of routine practice.
Your healthcare team will typically also order:
- Ferritin and transferrin saturation. These are markers of iron status. The European Society for Medical Oncology (ESMO) guidelines consider them necessary.
- If necessary, vitamin levels (B12, folic acid) and kidney function.
- If bleeding is suspected, gastrointestinal tests or a review of your medications.
How It Is Treated
There is no single treatment. The SEOM summarizes four pillars: addressing the underlying cause, erythropoiesis-stimulating agents, intravenous iron, and blood transfusions. Your care team will make the choice based on your symptoms, hemoglobin level, iron test results, and your situation (whether treatment is curative or palliative, your heart health, other medical conditions, etc.).
1. Address the underlying cause
If there is bleeding, it must be treated. If there are vitamin deficiencies, they are corrected. Surgery to remove the tumor can stop the blood loss, although it may take time for the anemia to resolve.
2. Iron
It can be administered orally (tablets or packets) or intravenously. According to the 2020 SEOM guidelines, oral or intravenous iron is recommended for absolute iron deficiency, and if there is no response after four weeks of oral treatment, the patient is switched to intravenous iron. In patients undergoing chemotherapy, a meta-analysis of randomized trials found that intravenous iron reduced the percentage of patients requiring transfusions compared to oral iron.
Some important points:
- Intravenous iron is administered in the outpatient clinic. It should not be given if the patient has an active infection. Patients with active infections should not receive it.
- Do not take iron on your own “just in case.” The SEOM guidelines recommend discontinuing all iron supplementation when ferritin and transferrin saturation levels are very high. Testing should be done first.
- Oral iron can darken stools and cause digestive discomfort. If you’re unsure whether your dark stools are due to iron or bleeding, ask your healthcare team.
3. Bone Marrow-Stimulating Drugs (Erythropoietin and Similar Drugs)
These are called “erythropoiesis-stimulating agents” (ESAs). They are versions of the hormone that signals the bone marrow to produce red blood cells. They are administered by injection. The SEOM recommends using them only for symptomatic anemia, as an alternative to transfusion, and in patients with a favorable benefit-to-risk ratio.
They are not suitable for everyone. Common side effects include venous thrombosis and hypertension; in patients who are not suitable candidates, an increased risk of thrombosis, tumor progression, and reduced survival has been observed. That is why your oncologist evaluates each case individually. The AEMPS product information sheet also warns that in some situations, blood transfusions should be the treatment of choice. If you use these medications, iron is usually taken alongside them: in cancer patients with low transferrin saturation, an iron supplement is recommended.
4. Blood Transfusion
This is the fastest option when anemia is severe or causes serious symptoms. The ESMO guidelines list rapid symptom relief as an advantage and transfusion reactions and circulatory overload as disadvantages. The decision depends on how you’re feeling and your lab results. There’s no one-size-fits-all magic number.
Summary of Options
| Option | What it’s for | Things to Keep in Mind |
|---|---|---|
| Oral iron | Iron deficiency | May cause digestive discomfort; if there is no response, intravenous administration should be considered |
| Intravenous iron | Iron deficiency, sometimes in combination with other treatments | Administered in the hospital; not for active infection |
| Bone marrow stimulants (ESAs) | Symptomatic anemia during chemotherapy | Risk of thrombosis; not suitable for everyone |
| Transfusion | Severe anemia or anemia with serious symptoms | Fast-acting; does not last as long as correcting the underlying cause |
When to notify your care team
With chemotherapy, your team will order tests before each cycle. Even so, let them know if you notice more fatigue or shortness of breath than usual. Cancer centers ask you to report it if, during treatment, you feel more tired than usual or have shortness of breath. Call your outpatient clinic or your oncologist if:
- You have red or black stools, or vomit that contains blood or looks like coffee grounds. ACS also includes dizziness or falls, and being unable to get out of bed for more than 24 hours.
- You are much paler, weaker, or more dizzy than you were the previous week.
- You have a fever (monitor your temperature according to your healthcare team’s instructions; in many hospitals, the threshold is 38 °C). This is an emergency in itself (see Guideline #43).
When to Go to the Emergency Room or Call 112
Go to the emergency room if you have unstoppable bleeding, new or worsening confusion, shortness of breath at rest, chest pain, or an irregular heartbeat. Do not wait until your next appointment. If possible, bring your chemotherapy report or your hospital’s on-call phone number.
What You Can Do
- Speak up. Tell your oncologist how you’re feeling, even if you think it’s “normal with chemo.”
- Ask for your test results (hemoglobin, ferritin) and write them down. You have the right to your medical records (Guide #36).
- Don’t self-medicate. Don’t take iron, vitamins, or “blood-boosting” products without first discussing it with your care team.
- Conserve your energy. Rest when you need to, get up slowly, and ask for help with heavy tasks.
- Eat as well as you can. A healthy diet helps, but on its own it won’t correct anemia caused by chemotherapy or bleeding. See guide #29 for more information.
One last thing
Anemia is treatable and doesn’t mean your chemotherapy isn’t working. Nor does it mean you’ve done anything wrong. If you feel weak, this is useful information for your care team and may change how they treat you. This guide provides general information and is not a substitute for your oncologist.