Macrocytic anemia is a condition where your red blood cells are larger than normal and you don’t have enough of them to deliver oxygen efficiently throughout your body. It’s identified on a standard blood test called a complete blood count (CBC), which measures something called mean corpuscular volume (MCV). A normal MCV falls between 80 and 100 femtoliters. When your MCV is above 100, your red blood cells are oversized, and you’re in macrocytic territory.
The condition has several possible causes, from vitamin deficiencies to alcohol use to thyroid problems, and the underlying reason matters because it determines both symptoms and treatment.
Why Red Blood Cells Grow Too Large
Red blood cells are produced in your bone marrow through a tightly coordinated process. As they develop, their DNA directs the cell to divide and mature on schedule. When something disrupts DNA production inside these developing cells, the inner machinery of the cell (the cytoplasm) keeps growing at a normal pace while the nucleus lags behind. The result is a mismatch: the cell’s outer structure balloons in size while its core can’t keep up. These oversized, poorly formed cells get released into your bloodstream, but they don’t carry oxygen as effectively as normal red blood cells do.
This isn’t just a matter of size. Your bone marrow also produces fewer functional cells overall when the process goes wrong, which is why macrocytosis often comes paired with anemia, meaning a low red blood cell count or low hemoglobin.
Megaloblastic vs. Non-Megaloblastic Types
Macrocytic anemia falls into two broad categories based on what’s going wrong at the cellular level.
Megaloblastic Anemia
This is the most well-known form, and it’s driven by a direct problem with DNA synthesis. Vitamin B12 and folate are both essential building blocks for DNA inside developing red blood cells. When either vitamin is deficient, cells can’t replicate their genetic material properly. The developing cells keep growing physically but can’t divide on time, producing the characteristic oversized cells called megaloblasts. A hallmark sign on blood work is hypersegmented neutrophils, a type of white blood cell with an abnormally shaped nucleus. B12 and folate deficiencies are also interconnected: when B12 is low, the body can’t properly activate folate, so a B12 shortage effectively creates a folate shortage too.
Non-Megaloblastic Anemia
In this form, DNA synthesis is intact. The red blood cells are still too large, but the cause lies elsewhere. Common triggers include chronic alcohol use, liver disease, and an underactive thyroid. Certain medications, particularly some chemotherapy drugs and anti-seizure medications, can also enlarge red blood cells. These conditions affect cell membranes or the bone marrow environment in ways that produce oversized cells without the DNA-related abnormalities seen in the megaloblastic type. A blood smear in non-megaloblastic cases typically lacks those hypersegmented neutrophils.
Common Causes
Vitamin B12 deficiency is one of the most frequent culprits. Your body absorbs B12 from animal-based foods like meat, eggs, and dairy, using a specialized protein in the stomach called intrinsic factor. People who follow strict vegan diets, those with conditions affecting the stomach or small intestine (such as celiac disease or Crohn’s disease), and older adults who produce less stomach acid are all at higher risk. An autoimmune condition called pernicious anemia destroys the cells that make intrinsic factor, blocking B12 absorption entirely.
Folate deficiency is another major cause. Folate is found in leafy greens, legumes, and fortified grains. Deficiency tends to develop faster than B12 deficiency because the body stores much less folate. Pregnancy significantly increases folate demand, and people with alcohol use disorder often have poor folate intake and absorption simultaneously.
Alcohol itself is one of the most common causes of macrocytosis, even without a clear vitamin deficiency. Alcohol has a direct toxic effect on the bone marrow and can alter the composition of red blood cell membranes, making them larger. Liver disease, which frequently accompanies heavy drinking, compounds the problem.
Hypothyroidism slows down metabolism throughout the body, including in the bone marrow, and can lead to enlarged red blood cells. Certain bone marrow disorders, including myelodysplastic syndromes, are less common but more serious causes.
How It Feels
Because macrocytic anemia reduces your blood’s oxygen-carrying capacity, the symptoms overlap with other forms of anemia. Fatigue is usually the first and most noticeable sign. You might feel short of breath during activities that didn’t used to wind you, notice your heart racing, or feel lightheaded when standing up quickly. Pale or slightly yellowish skin can develop, along with general weakness.
When B12 deficiency is the cause, the symptom picture expands into neurological territory. B12 plays a critical role in maintaining the protective coating around nerves, so a prolonged deficiency can cause tingling or numbness in the hands and feet, difficulty with balance and coordination, memory problems, and even mood changes like depression or irritability. A sore, smooth, red tongue (glossitis) is another classic finding. These neurological symptoms can become permanent if B12 deficiency goes untreated for a long time, which is why identifying the underlying cause matters so much.
Folate deficiency produces similar blood-related symptoms but generally doesn’t cause the nerve damage associated with B12 deficiency.
How It’s Diagnosed
The process typically starts with a CBC that shows an elevated MCV above 100 fL. From there, the goal is figuring out why the cells are enlarged. A peripheral blood smear, where a technician examines your blood under a microscope, helps distinguish between megaloblastic and non-megaloblastic causes. Hypersegmented neutrophils point strongly toward a B12 or folate problem.
Blood levels of B12 and folate are checked directly. Additional markers like methylmalonic acid and homocysteine can help pinpoint which vitamin is deficient when levels are borderline. A reticulocyte count, which measures how many young red blood cells your marrow is releasing, helps assess whether the bone marrow is responding appropriately. Thyroid function tests and liver panels round out the workup when vitamin levels come back normal.
In a large European hospital study of elderly patients, macrocytic anemia accounted for about 18% of all anemia cases in people 64 and older. Notably, a large proportion of those cases couldn’t be explained by B12 or folate deficiency alone, highlighting that the diagnostic process sometimes needs to go beyond basic vitamin levels. The study also found macrocytic anemia was more common in men than women in this age group.
Treatment Based on the Cause
When B12 deficiency is confirmed, supplementation corrects the problem in most cases. High-dose oral B12 (1,000 to 2,000 micrograms daily) works as well as injections for most people, which is good news if you’d rather take a pill than get regular shots. When absorption is the issue, as in pernicious anemia, or when neurological symptoms are present, injections are often used initially. British hematology guidelines recommend injections three times per week for two weeks in straightforward cases, and every other day for up to three weeks when nerve symptoms are involved.
Folate deficiency responds well to oral supplementation, and levels typically recover within a few weeks. One important caution: taking folate alone when B12 is actually the problem can improve the blood counts while masking ongoing nerve damage from the B12 shortage. That’s why testing for both vitamins before starting treatment is important.
For alcohol-related macrocytosis, reducing or stopping alcohol use allows the bone marrow to recover. Red blood cell size often normalizes within a few months of abstinence, though the timeline varies. If hypothyroidism is the cause, thyroid hormone replacement gradually brings cell size back to normal. Medication-induced macrocytosis may require switching to an alternative drug or simply monitoring if the benefits of the medication outweigh the blood changes.
Once treatment begins for B12 or folate deficiency, you can expect to feel noticeably better within a few weeks as new, properly sized red blood cells replace the old ones. Blood counts usually start improving within 7 to 10 days. Neurological symptoms from B12 deficiency improve more slowly, over weeks to months, and recovery depends on how long the deficiency lasted before treatment started.