What Causes Anemia? A Complete Guide

Anemia is one of the most common findings on a routine blood test, and it is best understood not as a single disease but as a description of a symptom: too few healthy red blood cells, or too little hemoglobin, to carry oxygen efficiently through the body. Because so many different conditions can lead to this same end result, figuring out the specific cause is the real starting point for treatment.

Iron-deficiency anemia is the most common type worldwide. It occurs when the body does not have enough iron to produce hemoglobin, often due to inadequate dietary intake, poor iron absorption, or ongoing blood loss such as heavy menstrual periods or slow gastrointestinal bleeding. Vitamin B12 deficiency and folate deficiency anemia occur when the body lacks these vitamins, which are needed to produce healthy red blood cells; causes include poor dietary intake, certain digestive conditions that impair absorption, and some medications.

Long-term conditions such as chronic kidney disease, rheumatoid arthritis, inflammatory bowel disease, and some cancers can cause anemia even when iron and vitamin levels are adequate. This happens because chronic inflammation interferes with how the body uses iron and can suppress red blood cell production in the bone marrow. Treating the anemia in these cases generally means managing the underlying chronic condition, sometimes alongside supportive measures.

Losing blood, whether suddenly from an injury or surgery, or gradually from sources such as heavy menstrual bleeding or slow gastrointestinal bleeding from an ulcer or other cause, reduces the number of red blood cells in circulation. Gradual blood loss often presents as iron-deficiency anemia over time, since the body's iron stores become depleted trying to keep up with red cell production.

Some anemias are genetic. Thalassemia is an inherited condition in which the body produces less or abnormal hemoglobin, ranging from mild forms that need little intervention to severe forms requiring regular blood transfusions. Sickle cell disease is another inherited condition, less common in this region, where red blood cells form an abnormal shape that can block blood flow. These conditions are usually identified through specific blood tests, sometimes including genetic testing, and often first suspected when routine anemia treatment does not work as expected.

Less commonly, anemia results from a problem in the bone marrow itself, either because it is not producing enough red blood cells (as in aplastic anemia) or because a bone marrow disorder, including some blood cancers, is crowding out normal red cell production. These causes are typically considered when anemia does not fit a nutritional or chronic-disease pattern, when other blood counts are also abnormal, or when standard treatment does not improve the anemia.

A complete blood count, including red cell size (MCV), is usually the first step and often narrows down the likely cause considerably, since different types of anemia tend to produce red cells of different sizes. Further tests may include iron studies, vitamin B12 and folate levels, kidney function tests, and, if a marrow-related cause is suspected, a bone marrow biopsy. Correctly identifying the cause matters because treatment varies widely, from dietary changes and supplements for nutritional anemia, to managing an underlying chronic disease, to more specialized care for inherited or bone marrow-related anemia.