Anemia happens when you have too few healthy red blood cells, or too little hemoglobin, to carry enough oxygen around the body. Its origins fall into three broad groups: blood loss, reduced red cell production, and increased red cell destruction. Its impact ranges from mild tiredness to breathlessness, heart strain, poor concentration, and, in children, delayed growth and development. Finding and treating the underlying cause is what makes the difference.
This article explains where anemia comes from, what it does to the body, how doctors work out the cause, and how it is treated. For a wider overview, see our anemia guide.
What Is Anemia?
Red blood cells are made in the bone marrow and circulate for about 120 days before being removed, mainly by the spleen. Each cell is packed with hemoglobin, the iron-containing protein that picks up oxygen in the lungs and releases it in the tissues.
Anemia is defined by a hemoglobin level below the normal range for a person’s age and sex. Commonly used adult cut-offs are shown below; exact reference ranges vary slightly between laboratories.
| Group | Anemia if hemoglobin is below |
|---|---|
| Adult men | 13 g/dL (130 g/L) |
| Adult non-pregnant women | 12 g/dL (120 g/L) |
| Pregnant women | 11 g/dL (110 g/L) |
| Children 6 months to 5 years | 11 g/dL (110 g/L) |
Anemia is a sign, not a diagnosis in itself. The key question is always why the hemoglobin is low.
The Origins of Anemia: Three Main Pathways
1. Blood loss
Sudden, heavy bleeding from trauma, surgery, or childbirth lowers red cell mass quickly. More often, the loss is slow and hidden: heavy menstrual periods, bleeding from the stomach or bowel (for example from ulcers, anti-inflammatory drugs, or bowel tumors), or frequent blood donation. Chronic blood loss drains the body’s iron stores, so it usually leads to iron deficiency anemia.
2. Reduced red cell production
The marrow needs raw materials and a healthy environment to make red cells. Production falls when:
- Iron is lacking, so hemoglobin cannot be built. This is the most common cause of anemia worldwide.
- Vitamin B12 or folate is lacking, which disrupts DNA synthesis and produces large, immature cells (megaloblastic anemia).
- Long-standing inflammation, infection, cancer, or autoimmune disease interferes with iron use and marrow response, causing anemia of chronic disease.
- The kidneys make too little erythropoietin, the hormone that signals the marrow to produce red cells, as happens in chronic kidney disease.
- The marrow itself fails or is replaced, as in aplastic anemia, myelodysplastic syndromes, leukemia, or marrow infiltration by other cancers.
3. Increased red cell destruction (hemolysis)
In hemolytic anemia, red cells are destroyed faster than the marrow can replace them. Causes may be inherited, such as sickle cell disease, thalassemia, hereditary spherocytosis, or G6PD deficiency, or acquired, such as autoimmune hemolytic anemia, certain infections, drugs, and mechanical damage from heart valves.
The Impact of Anemia on the Body
When oxygen delivery falls, the body compensates. The heart beats faster and pumps more blood per beat, and tissues extract more oxygen from each red cell. These adjustments explain many of the symptoms, and why slowly developing anemia may be surprisingly well tolerated until it is quite advanced.
- Fatigue and weakness — the most common complaint
- Pallor — pale skin, inner eyelids, and nail beds
- Shortness of breath, especially on exertion
- Palpitations or a racing heart
- Headaches, dizziness, and poor concentration
- Cold hands and feet
Some causes add their own features. Iron deficiency can cause brittle or spoon-shaped nails, a sore tongue, and cravings for ice (pica). B12 deficiency can cause numbness, tingling, balance problems, and memory changes. Hemolysis may cause jaundice and dark urine.
Longer-term effects
Untreated, significant anemia makes the heart work harder and can worsen angina or heart failure in people with existing heart disease. In pregnancy, anemia is associated with greater risks for both mother and baby. In infants and young children, iron deficiency can affect growth and brain development. In older adults, anemia is linked to falls, frailty, and reduced ability to manage daily life.
How Doctors Find the Cause
Diagnosis starts with a complete blood count (CBC), which measures hemoglobin, hematocrit, and red cell indices. The most useful index is the mean corpuscular volume (MCV), the average size of red cells, normally about 80 to 100 femtoliters.
| Red cell size | Typical causes |
|---|---|
| Small (microcytic, MCV below 80 fL) | Iron deficiency, thalassemia, some anemia of chronic disease |
| Normal (normocytic) | Acute blood loss, kidney disease, anemia of chronic disease, hemolysis, marrow failure |
| Large (macrocytic, MCV above 100 fL) | B12 or folate deficiency, alcohol, liver disease, hypothyroidism, myelodysplasia |
Further tests are chosen based on these results:
- Reticulocyte count — shows whether the marrow is responding. High counts suggest bleeding or hemolysis; low counts point to a production problem.
- Iron studies, including ferritin, and vitamin B12 and folate levels
- Peripheral blood smear — a microscope look at cell shapes
- Kidney and liver function, and markers of hemolysis
- Bone marrow examination or genetic testing when marrow failure or an inherited disorder is suspected
In children, an unexplained low count across several cell lines needs specialist review; our article on pediatric aplastic anemia explains one important cause.
Treatment: Matching the Cause
- Iron deficiency — oral or intravenous iron, plus finding and stopping any source of blood loss.
- B12 or folate deficiency — supplements; B12 is often given by injection when absorption is impaired, as in pernicious anemia.
- Anemia of chronic disease — treating the underlying condition; erythropoiesis-stimulating agents in selected patients, particularly with kidney disease.
- Autoimmune hemolysis — corticosteroids and other immune-suppressing treatments.
- Marrow failure and inherited disorders — transfusions, disease-specific therapy, and in some cases stem cell transplantation or gene therapy.
Blood transfusion is used for severe or symptomatic anemia but does not fix the cause. Iron should not be taken “just in case,” because unnecessary iron can build up and damage organs.
When to See a Doctor
See your doctor if you have ongoing tiredness, breathlessness, pale skin, or palpitations, particularly with heavy periods, black or bloody stools, or unexplained weight loss. Seek urgent care for chest pain, fainting, severe breathlessness at rest, or signs of heavy bleeding.
Frequently Asked Questions
What is the most common cause of anemia?
Iron deficiency is the most common cause worldwide. In adults it usually reflects blood loss, such as heavy periods or gastrointestinal bleeding, or poor absorption, rather than diet alone.
Can anemia go away on its own?
Mild anemia after a single bleed may correct itself as the marrow recovers, provided iron stores are adequate. Most anemia persists until the cause is identified and treated, so it is worth investigating.
Is anemia dangerous?
Mild anemia is often not dangerous in itself, but it can signal a serious underlying problem. Severe anemia strains the heart and can be life-threatening, especially in older people or those with heart disease.
Why does anemia make you tired?
With less hemoglobin, each heartbeat delivers less oxygen to muscles and the brain. The body compensates by working harder, which leaves less reserve for everyday activity and causes fatigue.
