Many types of cancer can cause low hemoglobin, but the ones most directly responsible fall into two broad categories: cancers that originate in the blood and bone marrow (like leukemia, lymphoma, and myeloma) and cancers that cause chronic internal bleeding (especially colorectal and stomach cancers). More than 60% of cancer patients have anemia at some point, and the risk climbs with more advanced disease. Understanding which cancers drive hemoglobin down, and how they do it, can help you make sense of lab results that feel alarming.
Blood and Bone Marrow Cancers
Cancers that start in the bone marrow are among the most direct causes of low hemoglobin because the marrow is where red blood cells are made. Leukemia, lymphoma, and multiple myeloma all interfere with this process, though each does it a bit differently.
In leukemia, abnormal white blood cells multiply rapidly and crowd out the cells responsible for producing red blood cells. The marrow essentially runs out of room to do its normal job. Lymphoma can have a similar effect when it infiltrates the marrow, and roughly 90% of patients with non-Hodgkin lymphoma develop some degree of anemia during their first round of chemotherapy, compounding the problem.
Multiple myeloma takes a particularly aggressive approach. Malignant plasma cells in the marrow overexpress certain proteins that actively trigger the death of immature red blood cell precursors. So it’s not just a space problem. The cancer cells are directly killing the cells that would have become healthy red blood cells, progressively exhausting the body’s ability to produce them.
Myelodysplastic Syndromes
Myelodysplastic syndromes (MDS) are a group of disorders in which the bone marrow produces blood cells that are abnormal in shape and function. Low hemoglobin is often the very first sign. In fact, at least one low blood count is required to make the diagnosis: below 12 g/dL for women or below 13 g/dL for men. Because MDS can eventually progress to acute leukemia, persistent unexplained anemia in older adults sometimes turns out to be an early warning of this condition.
Colorectal and Stomach Cancers
Gastrointestinal cancers are the classic example of tumors that cause anemia through chronic blood loss. Colorectal cancer, in particular, often bleeds slowly from the surface of the tumor as it ulcerates. The bleeding can be so gradual that you wouldn’t notice it in your stool, but over weeks and months it drains your iron stores. This leads to iron-deficiency anemia, the most common type of anemia in colorectal cancer patients.
The mechanism is twofold. The bleeding itself depletes your body’s iron supply (absolute iron deficiency). At the same time, inflammation from the cancer triggers a hormone called hepcidin that locks iron inside your cells, preventing it from being used to build new red blood cells (functional iron deficiency). Many patients have both problems simultaneously, which is why their hemoglobin can drop so significantly. Stomach cancer works through the same combination of bleeding and inflammation.
Solid Tumors That Spread to Bone Marrow
Cancers that didn’t originate in the marrow can still end up there. When tumor cells spread to the bone marrow through the bloodstream, they disrupt normal blood cell production in much the same way blood cancers do. This is called myelophthisic anemia.
Prostate cancer is the solid tumor most likely to metastasize to bone marrow, with about 36% of cases showing marrow involvement in one large study. Gastric cancer and melanoma tied for second at 25%. Breast cancer, lung cancer, thyroid cancer, and kidney cancer also frequently spread to the marrow. Among patients whose solid tumors had reached the marrow, anemia was the most common consequence, affecting over 70% of them.
Kidney Cancer
Kidney cancer deserves special mention because it has a unique relationship with hemoglobin. The kidneys produce erythropoietin, the hormone that signals the marrow to make red blood cells. You might expect kidney cancer to increase that signal, and in 1% to 5% of cases it actually does, causing abnormally high red blood cell counts. But roughly 35% of kidney cancer patients develop anemia instead. The inflammation caused by the tumor, combined with iron deficiency and possible inactivity of the erythropoietin being produced, overwhelms whatever extra hormone the cancer generates.
How Cancer Lowers Hemoglobin Beyond Bleeding
Even cancers that don’t bleed and don’t invade the marrow can cause low hemoglobin. This happens through a process called anemia of chronic disease, and it’s driven by the body’s inflammatory response to the cancer itself.
When a tumor triggers chronic inflammation, the immune system releases signaling molecules (particularly one called IL-6) that suppress hemoglobin production in multiple ways. They reduce the kidney’s output of erythropoietin. They directly block the growth and maturation of red blood cell precursors in the marrow. And they ramp up hepcidin, which traps iron inside storage cells so it can’t be used. On top of all that, inflammation shortens the lifespan of existing red blood cells, so you’re losing them faster while making fewer replacements.
In patients with solid tumors, hemoglobin levels correlate inversely with inflammatory markers, cancer stage, and performance status. The more advanced and aggressive the cancer, the more inflammation it generates, and the lower hemoglobin tends to fall.
Chemotherapy as a Compounding Factor
Cancer treatment itself frequently worsens anemia. Chemotherapy drugs damage rapidly dividing cells, and the blood-forming cells in bone marrow are among the most vulnerable. In a study of nearly 700 patients with non-Hodgkin lymphoma, about 37% developed moderate anemia (hemoglobin below 10 g/dL) during treatment, and nearly 12% developed severe anemia (below 8 g/dL). The risk of severe drops increased with each successive treatment cycle, peaking around the sixth round.
This means someone whose hemoglobin was borderline at diagnosis may see it fall significantly once treatment begins. The combination of the cancer itself and the treatment creates a double hit to red blood cell production.
How Cancer-Related Anemia Is Managed
Treatment depends on what’s driving the low hemoglobin. If iron deficiency is a factor, iron supplementation (either oral or intravenous) can help restore hemoglobin levels. Both forms are considered acceptable, and the choice typically comes down to how quickly levels need to rise, how well you tolerate oral iron, and what’s available.
For patients receiving chemotherapy, medications that stimulate red blood cell production may be added. Iron supplementation alongside these medications can further improve the response and reduce the need for blood transfusions. These stimulating agents are generally reserved for chemotherapy-related anemia, not other types of cancer-associated anemia. Blood transfusions remain an option when hemoglobin drops low enough to cause symptoms like severe fatigue, shortness of breath, or dizziness.
Treating the underlying cancer is ultimately the most important step. When the tumor shrinks or goes into remission, the inflammatory signals that suppress red blood cell production tend to ease, and hemoglobin often improves on its own.