Many types of cancer can cause low hemoglobin, but the ones most commonly responsible fall into three groups: cancers that cause internal bleeding (especially colorectal and stomach cancers), blood cancers that disrupt red blood cell production directly (like leukemia and multiple myeloma), and solid tumors that spread to the bone marrow (most often breast and prostate cancers). In cancer care, hemoglobin of 11 g/dL or below is considered anemia, with levels under 8 g/dL classified as severe.
Colorectal and Other GI Cancers
Colorectal cancer is one of the most recognized causes of cancer-related low hemoglobin, and sometimes anemia is the first sign that leads to diagnosis. Tumors in the colon or rectum often bleed slowly into the digestive tract in amounts too small to notice in the stool. Over weeks and months, this hidden blood loss drains your body’s iron stores, and without enough iron, your bone marrow can’t build red blood cells effectively. The result is iron deficiency anemia that may show up as fatigue, paleness, or shortness of breath long before any digestive symptoms appear.
Stomach and esophageal cancers work the same way. In studies of patients with unexplained hidden GI bleeding, 29 to 56 percent had a source in the upper digestive tract, including stomach tumors. In patients under 40, small bowel tumors are the most common cause of this type of occult bleeding. Because the blood loss is invisible, the general rule in medicine is that any man or postmenopausal woman who develops iron deficiency anemia should be evaluated for a GI source of bleeding until one is ruled out.
Among colorectal cancer patients specifically, about 35 percent have hemoglobin at or below 12 g/dL, and roughly 1 in 10 fall below 10 g/dL, the threshold where anemia starts to significantly affect daily functioning.
Blood Cancers: Leukemia, Lymphoma, and Myeloma
Blood cancers lower hemoglobin through a more direct route. Your bone marrow is the factory where red blood cells are made, and blood cancers essentially hijack that factory. In leukemia, abnormal white blood cells multiply uncontrollably and take up space that would normally be used to produce healthy red blood cells. The marrow becomes so overrun that it simply can’t keep up with your body’s need for new red blood cells.
Multiple myeloma causes a similar problem. Myeloma cells multiply within the bone marrow and crowd out healthy red blood cell production. But crowding isn’t the only issue. Even when myeloma cells haven’t physically filled the marrow, they interfere with the hormonal signals that normally tell the marrow to produce red blood cells. This means myeloma can suppress red blood cell output even in early stages. Anemia is one of the hallmark features of myeloma and is present in the majority of patients at some point during their disease.
Lymphomas, particularly aggressive or advanced forms, can also infiltrate the bone marrow and reduce hemoglobin. They may additionally trigger chronic inflammation that locks iron away in storage, making it unavailable for red blood cell production even when your body technically has enough iron.
Solid Tumors That Spread to Bone Marrow
When solid tumors metastasize to bone, the cancer cells can physically replace the marrow tissue responsible for making blood cells. This is sometimes called myelophthisic anemia, and it can cause hemoglobin to drop significantly. The cancers most likely to do this are breast cancer and prostate cancer, which have a strong tendency to spread to bone. Less commonly, kidney, lung, adrenal, and thyroid cancers can also invade the marrow.
This type of anemia tends to appear in later stages of disease, when cancer has already spread from its original site. It often shows up alongside other blood count abnormalities, since the marrow damage affects white blood cells and platelets too, not just red blood cells.
Lung and Breast Cancer
Lung cancer has one of the highest rates of anemia among solid tumors. In one large study across multiple tumor types, over 41 percent of lung cancer patients had hemoglobin at or below 12 g/dL, and about 17 percent had levels below 10 g/dL. That’s a notably higher rate than most other solid tumors. The causes are layered: lung cancer triggers significant systemic inflammation, which impairs iron availability and suppresses red blood cell production. Chemotherapy compounds the problem further.
Breast cancer causes anemia less frequently at diagnosis, with about 27 percent of patients showing low hemoglobin in the same study, and roughly 6 percent falling below 10 g/dL. However, those numbers climb during treatment, and they rise substantially in patients whose cancer has spread to the bones.
How Cancer Lowers Hemoglobin: The Main Mechanisms
Understanding the mechanism matters because it affects how the anemia gets treated. Cancer causes low hemoglobin through several overlapping pathways.
- Chronic blood loss: GI cancers, bladder cancers, and gynecologic cancers can bleed slowly, draining iron over time.
- Bone marrow invasion: Blood cancers and metastatic solid tumors physically replace the marrow tissue that makes red blood cells.
- Inflammation: Most cancers trigger an inflammatory response that traps iron inside cells and reduces the body’s production of the hormone (erythropoietin) that stimulates red blood cell creation.
- Nutritional deficiency: Cancers of the stomach, small intestine, or pancreas can impair absorption of vitamin B12 and folate, both essential for red blood cell formation. Surgical removal of parts of the stomach or intestine has the same effect.
- Chemotherapy: Many cancer treatments are toxic to the bone marrow and can significantly reduce hemoglobin during and after treatment cycles.
In many patients, more than one of these mechanisms is at work simultaneously, which is why cancer-related anemia can be stubborn to correct.
Severity Levels and What They Mean
Cancer care uses a standardized grading system for anemia. Mild anemia means hemoglobin between 10 and 11 g/dL. Moderate anemia falls between 8 and 10 g/dL. Severe anemia is anything below 8 g/dL. These thresholds matter because they guide treatment decisions.
At the mild stage, you might notice some extra tiredness but function relatively normally. Moderate anemia often brings more noticeable fatigue, difficulty concentrating, and shortness of breath with activity. Severe anemia can cause rapid heartbeat, dizziness, and significant limitations on what you can do physically. It may require a blood transfusion.
For patients receiving chemotherapy that isn’t intended to cure the cancer, treatment guidelines recommend considering medications that boost red blood cell production once hemoglobin drops below 10 g/dL. Blood transfusions are an option at any severity level depending on symptoms and how quickly the anemia developed.
Why Unexplained Low Hemoglobin Deserves Investigation
Low hemoglobin has many non-cancer causes, including heavy periods, poor diet, and chronic diseases like kidney failure. But when anemia appears without an obvious explanation, particularly iron deficiency anemia in men or postmenopausal women, it warrants a thorough workup. In prospective studies of patients evaluated for unexplained hidden GI bleeding, 20 to 30 percent turned out to have a colorectal source such as cancer or precancerous polyps.
The connection between unexplained anemia and cancer is strong enough that it sometimes serves as the earliest detectable sign of a tumor, months before other symptoms develop. This is especially true for right-sided colon cancers, which tend to bleed silently without causing changes in bowel habits that would otherwise prompt a visit to a doctor.