Large red blood cells mean your red blood cells are bigger than normal, a finding doctors call macrocytosis. It shows up on a standard blood test as an elevated MCV (mean corpuscular volume), which measures the average size of your red blood cells. Normal MCV falls between 80 and 100 femtoliters (fL). When your MCV is above 100 fL, your red blood cells are considered large, and the result typically triggers further investigation into the cause.
Why Red Blood Cell Size Matters
Red blood cells are built to be a specific size so they can move through tiny blood vessels and deliver oxygen efficiently. When something disrupts the way your bone marrow produces these cells, the cells can come out larger than they should be. The size itself isn’t the disease. It’s a signal pointing toward an underlying problem, whether that’s a vitamin deficiency, a medication side effect, or a more serious condition.
How high your MCV is can hint at what’s going on. A mildly elevated MCV between 100 and 110 fL is more likely caused by something benign. An MCV above 110 fL raises the possibility of a significant vitamin deficiency or a bone marrow disorder. Extreme elevations above 130 fL are uncommon and tend to be linked to certain medications, particularly some used to treat HIV.
Vitamin B12 and Folate Deficiency
The most well-known cause of large red blood cells is a shortage of vitamin B12 or folate (vitamin B9). Both nutrients are essential for building DNA inside developing blood cells. When either one is lacking, cells can’t divide properly during production in the bone marrow. The cell’s inner machinery falls out of sync: the outer shell keeps growing while the nucleus lags behind. The result is an oversized, oval-shaped red blood cell that doesn’t function as well as a normal one.
This type of enlargement is called megaloblastic anemia. On a blood smear, it has distinctive features. The red blood cells look oval rather than round, and certain white blood cells (neutrophils) develop extra segments in their nuclei. An MCV above 115 fL combined with these changes on a blood smear strongly suggests a megaloblastic process.
B12 deficiency can develop from a diet very low in animal products, from digestive conditions that block absorption (like celiac disease or Crohn’s disease), or from a condition called pernicious anemia where the stomach stops making a protein needed to absorb B12. Folate deficiency is less common today because many grain products are fortified, but it still occurs with poor dietary intake, heavy alcohol use, or during pregnancy when folate demand rises sharply.
Alcohol and Liver Disease
Alcohol is one of the most common causes of large red blood cells, and it works through more than one pathway. Heavy drinking directly affects how the bone marrow produces red blood cells, and it also damages the liver, which further disrupts blood cell formation. On top of that, people who drink heavily often have poor nutrition, including low folate levels, which compounds the problem.
In a study of over 300 people with alcohol-related liver disease, about 70% had macrocytosis. Nearly half had an MCV above 100 fL. By comparison, only 3% of people with liver disease from non-alcohol causes had an MCV that high. This makes elevated MCV a surprisingly reliable marker for alcohol-related liver problems. The finding was even more common in women, appearing in 86% of female participants versus 63% of males. Notably, only about 18% of the heavy drinkers with large red blood cells actually had low folate levels, meaning alcohol itself was the primary driver in most cases, not just the nutritional deficiency it causes.
Hypothyroidism
An underactive thyroid gland can also produce large red blood cells. Thyroid hormones play a role in stimulating red blood cell production, and when those hormone levels drop, the process slows and the cells that are produced may be abnormally large. This is worth knowing because thyroid-related macrocytosis won’t improve with B12 or folate supplements. If standard vitamin levels come back normal but your MCV remains high, thyroid function is one of the next things to check.
Medications That Raise MCV
Several medications can cause red blood cells to enlarge as a side effect. Hydroxyurea, a drug used for certain blood cancers and sickle cell disease, causes macrocytosis early in treatment, and the effect can look similar to a B12 deficiency on blood work. Some chemotherapy drugs, anti-seizure medications, and certain HIV antivirals (including zidovudine, stavudine, and lamivudine) are also well-established causes. If you’re taking any medication long-term and your blood work shows a high MCV, the drug itself may be the explanation.
Symptoms You Might Notice
Large red blood cells on their own don’t cause symptoms. What you feel depends on whether the enlargement has progressed to anemia, meaning your blood can no longer carry enough oxygen. When that happens, common symptoms include fatigue, weakness, pale skin, shortness of breath, and dizziness. These develop gradually, so many people don’t notice them until the anemia becomes moderate or severe.
B12 deficiency specifically can cause neurological symptoms that go beyond typical anemia: tingling or numbness in your hands and feet, difficulty with balance, memory problems, and mood changes. These can occur even before the anemia becomes obvious on blood work, which is why B12 levels are usually checked whenever large red blood cells show up.
How It Gets Investigated
When your MCV comes back high, the next step is usually a set of follow-up blood tests to narrow down the cause. These commonly include vitamin B12 and folate levels, thyroid function tests, liver function tests, and a reticulocyte count (which measures how quickly your body is producing new red blood cells). Your doctor may also order levels of methylmalonic acid and homocysteine, two compounds that rise when B12 or folate are low. These are more sensitive than measuring the vitamins directly.
A peripheral blood smear, where a lab technician examines your blood under a microscope, provides important clues. Oval-shaped large cells with extra-segmented white blood cells point toward a vitamin deficiency. Uniformly round large cells with normal variation in size suggest a non-megaloblastic cause like liver disease, alcohol use, or hypothyroidism. In some cases, a bone marrow biopsy is needed to rule out blood cancers or bone marrow disorders, but this is reserved for situations where simpler tests haven’t provided an answer.
How Large Red Blood Cells Are Treated
Treatment targets whatever is causing the enlargement. For B12 deficiency, supplementation through injections or high-dose oral supplements typically corrects the problem over several weeks to months. Folate deficiency responds to oral folate supplements, though it’s important to confirm that B12 isn’t also low, because folate supplementation alone can mask a B12 deficiency while neurological damage continues.
If alcohol is the cause, reducing or stopping alcohol intake allows MCV to normalize gradually, though this can take several months because red blood cells live for about 120 days. For hypothyroidism, thyroid hormone replacement resolves the macrocytosis as hormone levels return to normal. When a medication is responsible, your doctor may weigh whether the benefit of the drug outweighs the blood cell changes, since medication-induced macrocytosis is often harmless on its own and doesn’t always require a change in treatment.
The key point is that large red blood cells are a clue, not a diagnosis. The number on your blood work is telling you to look deeper, and in most cases, the underlying cause is treatable once identified.