Red blood cells can be low for three basic reasons: your body isn’t making enough of them, it’s destroying them too fast, or you’re losing them through bleeding. A normal count falls 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, and that’s when symptoms start showing up. The specific cause matters because it determines what needs to happen next.
Missing Nutrients Your Body Needs to Build Red Blood Cells
Your bone marrow, the spongy tissue inside large bones, is the factory that produces red blood cells. It needs three key raw materials: iron, vitamin B12, and folate. When any of these runs low, production slows down or the cells that are made don’t work properly.
Iron deficiency is the single most common cause of anemia worldwide. Without enough iron, bone marrow can’t produce hemoglobin, the protein inside red blood cells that actually carries oxygen. Heavy menstrual periods, pregnancy, and diets low in iron-rich foods are frequent culprits. But iron deficiency can also signal slow, hidden blood loss from the digestive tract, which is why doctors often investigate further rather than simply prescribing supplements.
Vitamin B12 and folate deficiencies work differently. Instead of producing too few cells, the bone marrow produces abnormally large red blood cells that can’t function properly. B12 deficiency is particularly tricky because some people absorb it poorly regardless of how much they eat, a condition called pernicious anemia. Beyond fatigue, B12 and folate deficiency can cause pins and needles in the hands and feet, muscle weakness, mouth ulcers, memory problems, and even confusion or depression. Left untreated, severe cases can damage the nervous system or lead to heart failure.
Chronic Blood Loss
Losing blood slowly over weeks or months drains the body’s red blood cell supply and, critically, depletes its iron stores. The bleeding is often invisible. Stomach ulcers, polyps in the colon, inflammatory bowel conditions, and even regular use of certain pain relievers can cause small but persistent bleeding in the digestive tract. You might not see any blood at all, yet your counts gradually drop.
For premenopausal women, heavy periods are one of the most common sources of ongoing blood loss. When monthly losses outpace the body’s ability to rebuild red blood cells, anemia develops over time. This is a situation where iron supplementation helps, but identifying and addressing the source of bleeding is just as important.
Chronic Diseases That Slow Production
Several long-term illnesses interfere with how the body makes red blood cells, even when nutrient levels are fine. Kidney disease is one of the clearest examples. Healthy kidneys produce a hormone called erythropoietin (EPO), which signals bone marrow to ramp up red blood cell production. As kidney function declines, EPO output drops, and the bone marrow simply doesn’t get the message to make enough cells. On top of that, low EPO triggers a chain reaction that locks iron away in storage, making it unavailable for building new cells.
Chronic infections, autoimmune diseases like rheumatoid arthritis or lupus, and cancer can all cause what’s broadly called “anemia of chronic disease.” The body’s inflammatory response redirects resources away from red blood cell production as part of its immune activity. This type of anemia is usually mild to moderate but persists as long as the underlying condition is active.
When the Body Destroys Its Own Red Blood Cells
Red blood cells normally live about 120 days before being recycled. In hemolytic anemia, they break down much sooner than that, and the bone marrow can’t replace them fast enough. This can happen for several reasons:
- Autoimmune reactions: The immune system mistakenly identifies red blood cells as foreign and attacks them.
- Inherited conditions: Genetic disorders like sickle cell disease, thalassemia, and G6PD deficiency produce red blood cells that are structurally fragile or misshapen, making them break apart more easily.
- Mechanical damage: Artificial heart valves, blood clots in small vessels, or other physical forces can shear red blood cells apart as they circulate.
- Medication reactions: Certain drugs can trigger the immune system to attack red blood cells. Common offenders include some antibiotics (particularly cephalosporins and penicillin derivatives), certain NSAIDs, and a few other medications like methyldopa and nitrofurantoin.
Bone Marrow Disorders
Sometimes the problem is the factory itself. In aplastic anemia, the bone marrow becomes damaged and stops producing enough blood cells of any type, not just red blood cells. This means white blood cell and platelet counts often drop too, raising the risk of infection and bleeding. Aplastic anemia is rare but serious, and it can be triggered by autoimmune attacks on the marrow, certain chemicals or medications, viral infections, or sometimes no identifiable cause at all.
Myelodysplastic syndromes are another group of bone marrow disorders where the marrow produces blood cells that are defective and die before maturing. Leukemia and other cancers that invade the bone marrow can crowd out normal blood cell production, causing red blood cell counts to fall alongside other abnormalities in a blood test.
What Low Red Blood Cells Feel Like
Mild drops in red blood cell count often produce no noticeable symptoms. As anemia worsens, the most common signs are fatigue and a general lack of energy that doesn’t improve with rest. This happens because your tissues aren’t receiving the oxygen they need to function.
Beyond tiredness, you might notice pale skin (especially noticeable in the nail beds, inner eyelids, and gums), shortness of breath during activities that didn’t used to wind you, a faster-than-usual heartbeat, dizziness when standing up, cold hands and feet, or headaches. The heart beats faster to compensate for fewer oxygen-carrying cells, which is why heart pounding or a racing pulse sometimes shows up before other symptoms do.
The specific cause of anemia can add its own symptoms on top of these general ones. B12 deficiency may bring neurological symptoms like numbness or cognitive changes. Hemolytic anemia can cause jaundice, a yellowish tint to the skin and eyes, because breaking red blood cells release pigments the liver has to process. Iron deficiency sometimes causes unusual cravings for ice, dirt, or starch.
How Doctors Figure Out the Cause
A standard complete blood count (CBC) is usually the first test that reveals low red blood cells, but it doesn’t explain why. The size and shape of the red blood cells offer important clues. Small, pale cells typically point to iron deficiency. Large cells suggest B12 or folate deficiency. Normal-sized cells that are simply too few in number lean toward chronic disease, kidney problems, or bone marrow issues.
From there, follow-up tests narrow things down: iron levels, B12 and folate levels, kidney function markers, and a reticulocyte count that measures how actively the bone marrow is producing new red blood cells. A high reticulocyte count means the marrow is working hard to replace cells being lost or destroyed. A low count means production itself is the problem. In some cases, a bone marrow biopsy is needed to look for disorders directly affecting the marrow.
Because the list of possible causes is long, the pattern of your blood work, your symptoms, your medical history, and sometimes your age and sex are all pieces that help pinpoint the answer. A 25-year-old woman with heavy periods and small, pale red blood cells has a very different workup ahead of her than a 65-year-old man with the same low count but normal-sized cells.