Anemia happens when your blood doesn't carry enough oxygen to your tissues

Anemia occurs when you have too few red blood cells or when the red blood cells you do have can't carry oxygen effectively. Red blood cells contain a protein called hemoglobin, which binds to oxygen in your lungs and releases it as blood circulates through your body. When hemoglobin levels drop below normal, your tissues don't get the oxygen they need to function properly. This shortage is what causes the symptoms most people with anemia experience.

The severity of anemia depends on how low your hemoglobin has fallen and how quickly it dropped. A mild case might cause fatigue you notice only during exercise. Severe anemia can make climbing stairs feel impossible or cause chest pain and shortness of breath at rest. Your doctor measures hemoglobin in grams per deciliter of blood (g/dL). For adult men, normal is roughly 13.5 to 17.5 g/dL; for adult women, roughly 12 to 15.5 g/dL, though these ranges vary slightly between labs.

Key Takeaways

  • Anemia means your blood carries less oxygen than normal, either because you have fewer red blood cells or because those cells don't work properly.
  • The most common cause is iron deficiency, but anemia can also result from vitamin B12 or folate shortage, chronic disease, blood loss, or bone marrow problems.
  • Symptoms include fatigue, shortness of breath, dizziness, pale skin, and cold hands or feet, though mild anemia may cause no symptoms at all.
  • A simple blood test called a complete blood count (CBC) measures hemoglobin and red blood cell count and is how anemia is diagnosed.
  • Treatment depends on the underlying cause—iron supplements for iron deficiency, B12 injections for pernicious anemia, or addressing the disease driving the anemia.

Why red blood cells fail or become too few

Your bone marrow produces roughly 2 million red blood cells every second. These cells live about 120 days before your spleen removes them and your body recycles the iron. Anemia develops when production slows, when cells die faster than normal, or when you lose blood faster than you can replace it.

Iron deficiency is the most common cause worldwide. Iron is essential for hemoglobin production. Without enough iron, your bone marrow can't make hemoglobin-rich cells. This happens through heavy menstrual bleeding, gastrointestinal bleeding (often from ulcers or inflammatory bowel disease), pregnancy, or simply not eating enough iron-rich foods. Vegetarians and vegans face higher risk because plant-based iron is absorbed less efficiently than iron from meat.

Vitamin B12 deficiency and folate deficiency prevent your bone marrow from making red blood cells the right size and shape. B12 comes mainly from animal products; vegans need supplements or fortified foods. Folate comes from leafy greens and legumes. Certain medications, digestive disorders like celiac disease, and a condition called pernicious anemia (where your stomach can't absorb B12) all cause these deficiencies.

Chronic disease causes anemia in a different way. Kidney disease, cancer, rheumatoid arthritis, and long-term infections reduce production of erythropoietin, a hormone that signals bone marrow to make red blood cells. Inflammation from these diseases also shortens red blood cell lifespan.

Bone marrow disorders like aplastic anemia, myelodysplastic syndromes, and leukemia directly damage the tissue that produces blood cells. These are less common but more serious.

Symptoms you might notice

Mild anemia often causes no symptoms at all—you might discover it only during a routine blood test. As anemia worsens, symptoms appear because your tissues are working harder to extract oxygen from blood that carries less of it.

The most common symptom is fatigue that doesn't improve with rest. You might feel exhausted after activities that never bothered you before. Shortness of breath happens because your lungs work harder to pull in oxygen. Dizziness or lightheadedness occurs because your brain isn't getting enough oxygen. Some people notice pale skin, pale coloring inside the lower eyelid, or pale nail beds. Cold hands and feet happen because your body prioritizes sending blood to vital organs. Headaches, difficulty concentrating, and chest pain or irregular heartbeat can occur in more severe cases.

Symptoms develop gradually in chronic anemia, so you might not notice them until they're significant. Anemia from sudden blood loss causes symptoms much faster and more dramatically.

How doctors diagnose anemia

Diagnosis starts with a blood test called a complete blood count (CBC). This test measures hemoglobin level, the number of red blood cells, and the size and shape of those cells. If hemoglobin is below normal, you have anemia. The CBC also measures white blood cells and platelets, which can point toward bone marrow problems.

Once anemia is confirmed, your doctor orders additional tests to find the cause. A peripheral blood smear lets a technician look at red blood cells under a microscope to check their shape and size. Iron studies measure iron, ferritin (stored iron), and transferrin (the protein that carries iron). B12 and folate levels are checked if those deficiencies are suspected. If kidney disease or chronic illness seems likely, your doctor checks kidney function and inflammatory markers.

If initial tests don't explain the anemia, or if bone marrow disease is suspected, your doctor may order a bone marrow biopsy—a procedure where a small sample of bone marrow is removed and examined under a microscope. This is less common but necessary when other tests are inconclusive.

How anemia is treated

Treatment targets the underlying cause rather than just raising hemoglobin numbers. Iron deficiency anemia is treated with iron supplements, usually taken by mouth. Ferrous sulfate is the most common form. It takes weeks to months for hemoglobin to normalize because your bone marrow needs time to produce new red blood cells. If you can't tolerate oral iron or have severe deficiency, iron can be given by injection.

B12 deficiency from pernicious anemia or veganism is treated with B12 injections or high-dose oral supplements, since the absorption problem means regular dietary B12 won't work. Folate deficiency is treated with folate supplements by mouth. Both take weeks to work.

Anemia from chronic disease is harder to treat. If kidney disease is the cause, erythropoietin-stimulating agents (medications that mimic the hormone your kidneys should be making) may be used. If inflammation is driving the anemia, treating the underlying disease—controlling rheumatoid arthritis, for example—can improve blood counts.

Severe anemia may require a blood transfusion to raise hemoglobin quickly, especially if you're having chest pain, severe shortness of breath, or other urgent symptoms. Transfusions are temporary; they don't treat the cause.

When anemia becomes serious

Mild to moderate anemia is usually manageable and doesn't cause permanent damage if treated. Severe untreated anemia can strain your heart. Your heart pumps faster to move oxygen-poor blood around your body, and over time this extra work can weaken the heart muscle and lead to heart failure.

Certain types of anemia carry specific risks. Sickle cell disease causes red blood cells to become crescent-shaped and get stuck in blood vessels, blocking blood flow and causing pain and organ damage. Hemolytic anemia, where red blood cells are destroyed faster than normal, can cause jaundice and kidney damage. Aplastic anemia, where bone marrow stops producing blood cells, is life-threatening without treatment.

Most anemia develops slowly enough that your body adapts. You feel worse before you feel better, but with treatment, hemoglobin rises and symptoms fade over weeks to months.

Frequently Asked Questions

Can anemia go away on its own?

Mild anemia from temporary blood loss or a short-term nutritional shortage might improve without treatment if the cause resolves. But most anemia requires treatment. Iron deficiency won't correct itself without iron supplementation, and B12 deficiency from pernicious anemia won't improve without B12 injections.

Is anemia the same as low iron?

No. Low iron is one cause of anemia, but not the only one. You can have low iron without anemia if your body hasn't yet produced enough hemoglobin-deficient cells. You can also have anemia from B12 deficiency, chronic disease, bone marrow problems, or rapid blood loss—none of which involve low iron.

Can you have anemia and not know it?

Yes. Mild anemia often causes no symptoms, especially if it develops slowly. Many people discover anemia during a routine blood test for an unrelated reason. Symptoms become noticeable as hemoglobin drops further or as anemia develops quickly.

Does anemia run in families?

Some types do. Sickle cell disease, thalassemia, and hereditary spherocytosis are inherited. Pernicious anemia has a genetic component. But iron deficiency anemia, the most common type, is usually caused by diet, blood loss, or absorption problems rather than genetics.

How long does it take to recover from anemia?

Recovery depends on the cause and severity. Iron deficiency anemia typically improves within weeks to months of starting supplements. B12 deficiency takes weeks to months. Anemia from chronic disease improves only if the underlying disease improves. Severe anemia may take longer because your bone marrow needs time to produce new red blood cells.