A low red blood cell count, called anemia, happens when your body either isn’t making enough red blood cells, is destroying them too quickly, or is losing them through bleeding. Normal ranges fall between 4.7 and 6.1 million cells per microliter for men and 4.2 to 5.4 million for women. When your count drops below these ranges, less oxygen reaches your tissues, which is why fatigue, weakness, and shortness of breath are the hallmark symptoms.
The causes range from something as straightforward as a nutritional deficiency to complex bone marrow disorders. Understanding which category your anemia falls into matters because the path back to a normal count depends entirely on what’s driving it down.
Nutritional Deficiencies
Iron, vitamin B12, and folate are the three nutrients your body needs most to build red blood cells, and a shortage of any one of them is the most common reason for a low count worldwide. Iron is a core component of hemoglobin, the protein inside red blood cells that carries oxygen. Without enough iron, your bone marrow produces smaller, paler cells that carry less oxygen than normal.
B12 and folate work differently. Both are essential for DNA synthesis inside developing red blood cells. When either is lacking, the cells can’t divide properly and instead grow abnormally large, a condition called megaloblastic anemia. These oversized cells are fragile, function poorly, and die sooner than they should. Notably, the effects of B12 and folate deficiency aren’t limited to blood. Symptoms like numbness, tingling, and cognitive changes can appear before any blood test looks abnormal, because these vitamins play roles throughout the nervous system and other organs.
Chronic Blood Loss
Losing blood gradually over weeks or months drains your iron stores without obvious warning signs. The most common sources are gastrointestinal bleeding and heavy menstrual periods. A slow-bleeding stomach ulcer, a colon polyp, or even long-term use of certain pain relievers that irritate the stomach lining can cause enough hidden blood loss to pull your red blood cell count down steadily.
This type of anemia is tricky because the bleeding is often invisible. You may not see blood in your stool, and the fatigue creeps in so gradually that you adjust to it. Doctors typically check for occult (hidden) blood in the stool and, if positive, follow up with imaging of the upper and lower digestive tract to find the source.
Kidney Disease and Chronic Illness
Your kidneys produce a hormone called erythropoietin, which signals your bone marrow to make red blood cells. In chronic kidney disease, the kidneys lose the ability to produce adequate amounts of this hormone. People with normal kidney function who become anemic will ramp up erythropoietin production by 10 to 100 times the usual level. Kidneys damaged by chronic disease simply can’t match that response, so the bone marrow never gets the signal to increase production.
Kidney disease also raises levels of hepcidin, a liver hormone that controls how much iron your body absorbs and recycles. Elevated hepcidin effectively locks iron away, making it unavailable for red blood cell production even when your total iron stores appear adequate. This combination of low erythropoietin and restricted iron access makes anemia one of the most common complications of advanced kidney disease, particularly in patients on dialysis.
Other chronic conditions, including autoimmune diseases, long-term infections, and cancer, trigger a similar pattern. Ongoing inflammation raises hepcidin and suppresses the bone marrow’s ability to respond to red blood cell demand.
Red Blood Cell Destruction
Sometimes the problem isn’t production but destruction. Hemolytic anemia occurs when red blood cells are broken down faster than the bone marrow can replace them. The causes split into two broad categories: problems inside the red blood cell itself, and external forces attacking otherwise normal cells.
On the external side, the immune system can mistakenly produce antibodies that tag red blood cells for destruction. This autoimmune hemolytic anemia can occur on its own or alongside other autoimmune conditions. Infections also destroy red blood cells directly. Malaria is the most significant example globally, with the parasite invading and rupturing red blood cells as part of its life cycle. Babesiosis, a tick-borne infection, works similarly.
Physical trauma to red blood cells is another external cause. Mechanical heart valves, certain circulatory support devices, and conditions that damage small blood vessels can shear red blood cells apart as they flow through the bloodstream.
Inherited Blood Disorders
Several genetic conditions affect either the structure or production of hemoglobin, leading to lifelong reductions in red blood cell count. The two most significant groups are sickle cell disorders and thalassemias.
In sickle cell disease, a single gene mutation causes hemoglobin to form rigid, crescent-shaped cells under low-oxygen conditions. These misshapen cells get stuck in small blood vessels and break down far sooner than normal red blood cells, which typically live about 120 days. Sickle cells may survive only 10 to 20 days, creating a constant deficit the bone marrow struggles to keep up with.
The thalassemias are caused by reduced production of one of the protein chains that make up hemoglobin. In beta-thalassemia, excess protein chains accumulate inside developing red blood cells in the bone marrow, causing them to self-destruct before they ever enter the bloodstream. The severity varies widely. Mild forms cause only slight anemia, while severe forms require regular blood transfusions to maintain adequate red blood cell levels. Alpha-thalassemia follows a similar pattern, with severity depending on how many of the four alpha-globin genes are affected.
G6PD deficiency, an X-linked inherited condition, takes a different path. Red blood cells function normally most of the time but are vulnerable to oxidative stress. Certain foods (fava beans are the classic trigger), medications, and infections can set off sudden episodes of hemolysis in people with this deficiency.
Bone Marrow Disorders
The bone marrow is the factory where all blood cells are made, and diseases that damage or crowd out this factory directly reduce red blood cell production. Myelodysplastic syndromes are a group of conditions in which the bone marrow produces defective blood cells due to acquired genetic mutations. The precursor cells either die in the marrow or fail to mature properly, a process called ineffective erythropoiesis, where the marrow is active but its output is poor.
Leukemia and other blood cancers can overtake the bone marrow with abnormal cells, leaving less room for normal red blood cell development. Aplastic anemia, a rarer condition, occurs when the bone marrow stops producing enough of all blood cell types. Cancers that originate elsewhere in the body can also spread to the bone marrow and disrupt production.
Medications That Lower Red Blood Cell Count
Certain drugs can trigger the immune system to attack red blood cells. Cephalosporins, a widely prescribed class of antibiotics, are the most common cause of drug-induced hemolytic anemia. Penicillin and its derivatives, some NSAIDs, and several other medications including levodopa (used for Parkinson’s disease), nitrofurantoin (a urinary tract antibiotic), and dapsone can also provoke this reaction.
Chemotherapy drugs are another major cause, though through a different mechanism. Rather than triggering immune destruction, they suppress bone marrow activity broadly, reducing production of red blood cells along with white blood cells and platelets. This is an expected side effect rather than an unusual reaction, and blood counts are monitored closely during treatment for this reason.
How Severity Is Classified
Anemia severity is measured by hemoglobin level rather than red blood cell count alone, because hemoglobin reflects oxygen-carrying capacity more directly. For men, mild anemia falls between 11 and 12.9 g/dL, moderate between 8 and 10.9 g/dL, and severe below 8 g/dL. For non-pregnant women, mild anemia is 11 to 11.9 g/dL, with moderate and severe at the same thresholds as men. Pregnant women have slightly lower cutoffs, with severe anemia defined as below 7 g/dL.
Mild anemia often produces no noticeable symptoms, or symptoms so subtle you attribute them to poor sleep or stress. Moderate anemia typically brings persistent fatigue, pallor, and shortness of breath with exertion. Severe anemia can cause rapid heart rate, dizziness, and chest pain, because the heart is working harder to compensate for reduced oxygen delivery. How quickly the count dropped matters too. A gradual decline over months gives your body time to adapt, so you may tolerate surprisingly low levels. A rapid drop from acute bleeding or sudden hemolysis tends to produce more dramatic symptoms at higher counts.