It is possible for cancer to cause low red blood cells, but low red blood cells are not necessarily cancer. There are three common causes of low red blood cells, including decreased production, excessive destruction and excessive loss. 1. Reduced red blood cell production: including aplastic anemia, pure red blood cell aplastic anemia, anemia caused by chronic diseases, anemia caused by tumor cells. 2. Excessive destruction of red blood cells: hemolytic anemia caused by various reasons, hypersplenism and so on. 3. Excessive loss of red blood cells: acute and chronic blood loss caused by various reasons, such as hemorrhoids, bleeding caused by gastrointestinal tumors, excessive menstrual flow during female menstruation, and some blood loss caused by various kinds of traumas and so on. Cancer can cause low red blood cell index due to tumor invasion of bone marrow and advanced malignant disease, but not all low red blood cell is caused by cancer. In conclusion, there are many reasons for low red blood cells, and it is recommended to go to the hospital to receive regular examination and treatment under the guidance of physicians.