Iron deficiency anemia is one of the most common diseases in hematology, and it is also the most neglected disease by patients. The diagnosis of iron-deficiency anemia is generally not difficult, as routine blood tests indicate microcytic hypochromic anemia, reduced serum ferritin level, and bone marrow aspiration indicates reduced or absent iron inside and outside the bone marrow, and the diagnosis is clear when the above indicators are met. The treatment is usually oral, intramuscular or intravenous iron supplementation, combined with the application of folic acid and vitamin C. However, can iron deficiency anemia be cured by iron supplementation alone? The answer is no. In the clinic, I usually give patients an analogy: the red blood cells (or iron reserves) in the body of a patient with iron deficiency anemia are like water in a barrel, and a leaky barrel leads to the loss of red blood cells, which in turn leads to anemia. Iron supplementation therapy usually only adds water to the barrel, but if we do not find the place where the barrel is leaking, even if we can fill the barrel temporarily, the water will still leak out after a long time. Therefore, iron deficiency anemia, besides iron supplementation treatment, finding and discovering the cause of iron loss (usually the cause of red blood cell loss) is an important part of its treatment! The causes of iron deficiency anemia formation are mainly in two general directions: one is insufficient absorption of iron and the other is excessive loss of iron. The first reason is very simple, because most Chinese people use iron pots to stir-fry, and there is enough iron in food, so there is often no problem of “eating less”. The iron eaten is mainly processed by stomach acid and absorbed in the upper part of the small intestine, and usually patients with major gastrectomy are prone to impaired iron absorption. The second cause is more complicated, as red blood cell loss can occur in almost all parts of the body, which requires individualized search for the cause of red blood cell loss. Usually iron deficiency anemia in young men often requires attention to digestive problems such as gastric ulcers and bleeding hemorrhoids; in young women, the focus is on causes such as abnormal menstrual flow. In contrast, older men and menopausal women need to focus on ruling out gastrointestinal tumors and gynecological tumors. For patients living in rural and pastoral areas, parasitic infections, such as hookworm infection, need to be excluded. In the case of women during pregnancy, iron deficiency anemia may also result due to the increased demand for iron. Of course, not all iron-deficiency anemia must have the whole body examined, but gastroscopy and gynecological examination are necessary for patients suspected of having gastrointestinal bleeding, menstrual bleeding or other conditions. Clinically, we often encounter cases of iron-deficiency anemia where malignant tumors are found by gastrointestinal examinations.