The most common causes of anemia

Anemia happens when your body does not have enough healthy red blood cells to carry oxygen to your tissues. The reason depends on whether your body is not making enough red blood cells, losing them too quickly, or destroying them. The three broadest categories are blood loss, reduced production, and increased destruction — and most causes of anemia fit into one of these.

Blood loss is the most straightforward: you lose red blood cells faster than your body replaces them. This can happen acutely (a single large bleed) or chronically (small bleeds over weeks or months). Reduced production means your bone marrow is not making enough new red blood cells, usually because it lacks the raw materials or the signals to do so. Increased destruction means your immune system or another process is breaking down red blood cells before their normal 120-day lifespan ends.

Key Takeaways

  • Iron deficiency is the most common cause of anemia worldwide, usually from chronic blood loss or inadequate dietary intake.
  • Vitamin B12 and folate deficiency prevent your bone marrow from making red blood cells properly, even when iron is present.
  • Chronic diseases like kidney disease and autoimmune conditions can cause anemia by reducing hormone signals or destroying red blood cells.
  • Heavy menstrual bleeding, ulcers, and colorectal polyps are frequent sources of ongoing blood loss that lead to anemia.
  • Some causes of anemia develop slowly over months and may not produce obvious symptoms until the shortage becomes severe.

Iron deficiency and chronic blood loss

Iron deficiency anemia develops when your body does not have enough iron to build hemoglobin, the protein in red blood cells that carries oxygen. Your body stores iron in the liver, spleen, and bone marrow, and you absorb more from food — but if you lose blood regularly, those stores deplete faster than they refill.

The most common sources of chronic blood loss are heavy menstrual periods (the leading cause in women of reproductive age), peptic ulcers, and colorectal polyps or tumors. Gastrointestinal bleeding can be obvious (blood in stool) or hidden (microscopic bleeding you cannot see). Men and postmenopausal women who develop iron deficiency anemia almost always have a source of bleeding in the digestive tract that needs investigation.

Iron deficiency can also develop without blood loss if you do not eat enough iron-rich foods or if your body cannot absorb iron properly. Celiac disease and Crohn's disease damage the intestines in ways that block iron absorption. Pregnancy increases iron demand because blood volume expands and the fetus needs iron too.

Vitamin B12 and folate deficiency

Vitamin B12 and folate are essential for your bone marrow to produce new red blood cells. Without enough of either, your marrow makes fewer cells, and the ones it does make are often larger and less functional than normal. This type of anemia is called megaloblastic because the abnormal cells are visibly larger under a microscope.

B12 deficiency usually comes from one of two sources: you do not eat enough (rare, since B12 is in meat, dairy, and eggs) or your body cannot absorb it. Pernicious anemia, an autoimmune condition, destroys cells in your stomach lining that produce intrinsic factor, a protein needed to absorb B12. Gastric surgery, Crohn's disease, and certain medications can also impair B12 absorption. Vegans and vegetarians who do not supplement are at risk because plant foods contain little or no B12.

Folate deficiency is usually dietary — it comes from not eating enough leafy greens, legumes, or fortified grains. Pregnancy increases folate demand. Alcohol use disorder can cause folate deficiency because alcohol damages the intestines and people with alcohol use disorder often eat poorly. Some medications, including methotrexate and certain anticonvulsants, interfere with folate metabolism.

Chronic disease and kidney problems

Long-standing illness can cause anemia even when iron, B12, and folate are adequate. Chronic kidney disease is a major example: your kidneys produce erythropoietin (EPO), a hormone that signals your bone marrow to make red blood cells. When kidney function declines, EPO production drops, and your marrow slows production.

Other chronic conditions that cause anemia include rheumatoid arthritis, lupus, inflammatory bowel disease, and cancer. These conditions trigger inflammation, which suppresses bone marrow production and can also shorten red blood cell lifespan. Severe infections, heart disease, and liver disease can all contribute to anemia as well. The anemia of chronic disease typically develops slowly and is usually mild to moderate unless the underlying condition is severe.

Autoimmune destruction and hemolytic anemia

Hemolytic anemia occurs when your immune system or another process destroys red blood cells faster than your bone marrow can replace them. In autoimmune hemolytic anemia, your body produces antibodies that attack your own red blood cells, marking them for destruction by the spleen.

Autoimmune hemolytic anemia can develop on its own or alongside other autoimmune diseases like lupus or rheumatoid arthritis. Some medications, including certain antibiotics and blood pressure drugs, can trigger it. Infections like mononucleosis and mycoplasma can also cause temporary hemolytic anemia.

Other causes of hemolytic anemia include hereditary conditions (sickle cell disease and hereditary spherocytosis are two examples), incompatible blood transfusions, and severe burns. Mechanical damage — from artificial heart valves or severe exercise — can also break red blood cells. Hemolytic anemia often develops acutely and produces more obvious symptoms like dark urine and jaundice because the breakdown of hemoglobin releases bilirubin.

Bone marrow disorders

Your bone marrow is where red blood cells, white blood cells, and platelets are made. If the marrow itself is damaged or diseased, production of all three cell types can suffer. Aplastic anemia occurs when the marrow stops producing enough cells of any type — this can follow chemotherapy, radiation, or certain medications, or it can develop without a clear trigger.

Myelodysplastic syndromes are a group of disorders in which the marrow produces cells that are abnormal or do not function properly. They are more common in older adults and can progress to leukemia. Leukemia itself crowds out normal marrow production. Severe infections and some medications can temporarily suppress marrow function.

Inherited conditions affecting red blood cells

Some people are born with conditions that affect how red blood cells are made or how long they survive. Sickle cell disease is an inherited disorder in which hemoglobin is abnormal, causing red blood cells to become rigid and sickle-shaped. These cells get stuck in blood vessels, causing pain and organ damage, and they break down much faster than normal cells.

Thalassemia is another inherited condition affecting hemoglobin production. People with thalassemia make red blood cells that are smaller and more fragile than normal, leading to hemolysis and anemia. Hereditary spherocytosis causes red blood cells to be round instead of disc-shaped, making them more fragile and prone to destruction by the spleen.

G6PD deficiency is an inherited enzyme deficiency that makes red blood cells vulnerable to breaking down when exposed to certain medications, infections, or foods like fava beans. These inherited conditions are present from birth, though symptoms may not appear until later in life or until a trigger (like infection or medication) causes hemolysis.

Frequently Asked Questions

Can anemia be caused by not eating enough?

Yes, but usually only for specific nutrients. Iron deficiency from low dietary intake is uncommon in developed countries but occurs in some populations. B12 deficiency from diet alone is rare unless you are vegan and do not supplement. Folate deficiency from poor diet is more common, especially in people with alcohol use disorder or those who eat very few vegetables or legumes.

Does anemia always cause symptoms?

No. Mild anemia may produce no symptoms at all, especially if it develops slowly and your body has time to adjust. Symptoms like fatigue, shortness of breath, and dizziness typically appear when anemia is moderate or severe, or when it develops rapidly. Some people discover anemia only through a blood test done for another reason.

Can medications cause anemia?

Yes. Some medications suppress bone marrow production (chemotherapy, certain antibiotics). Others interfere with nutrient absorption (metformin can reduce B12 absorption). Some trigger autoimmune hemolytic anemia (penicillin, sulfonamides). If you develop anemia after starting a new medication, tell your doctor — the cause may be the drug itself.

Is anemia from blood loss always obvious?

No. Bleeding in the digestive tract can be microscopic and invisible to you, but it still causes iron deficiency over weeks or months. Heavy menstrual bleeding is obvious to some women but not others. A blood test is the only way to know whether you are losing blood chronically.

Can anemia go away on its own?

It depends on the cause. Anemia from a temporary infection or medication may resolve once the trigger is gone. Anemia from iron, B12, or folate deficiency improves with supplementation. Anemia from chronic kidney disease, autoimmune disease, or inherited conditions usually requires ongoing treatment but may improve if the underlying cause improves.