Megaloblastic anemia is caused by impaired DNA synthesis in developing red blood cells, most often due to a deficiency of vitamin B12 or folate. These two nutrients are essential for cells to copy their DNA properly, and when either is missing, the bone marrow produces abnormally large, immature red blood cells that can’t function normally. Less commonly, certain medications that interfere with DNA synthesis can trigger the same problem.
How Impaired DNA Synthesis Creates the Problem
Both vitamin B12 and folate play critical roles in building DNA. When either nutrient is lacking, cells that divide rapidly are the first to suffer, and few cells in the body divide faster than the precursors to red blood cells in your bone marrow. These precursor cells keep growing in size because the rest of the cell matures on schedule while the nucleus lags behind, unable to divide properly. The result is oversized red blood cells called megaloblasts.
This mismatch between the nucleus and the rest of the cell shows up clearly on a blood test. Normal red blood cells measure between 80 and 100 femtoliters in volume. In megaloblastic anemia, that number climbs above 100, and a measurement above 115 femtoliters is particularly suggestive of B12 or folate deficiency. Another hallmark is hypersegmented neutrophils, a type of white blood cell whose nucleus develops extra lobes when DNA synthesis stalls. Finding these on a blood smear alongside oversized red blood cells is a strong signal that megaloblastic anemia is the cause.
Vitamin B12 Deficiency
Vitamin B12 deficiency is the most clinically significant cause of megaloblastic anemia because, unlike folate deficiency, it can also damage the nervous system. Your body stores enough B12 to last several years, so deficiency develops slowly and often goes unnoticed until it’s advanced.
Pernicious Anemia
The most well-known cause of B12 deficiency is pernicious anemia, an autoimmune condition in which your immune system attacks the stomach cells that produce intrinsic factor. Intrinsic factor is a protein you need to absorb B12 from food in your small intestine. Without it, virtually no dietary B12 gets into your bloodstream no matter how much you eat. Pernicious anemia is more common in older adults and often runs in families. A blood test for anti-intrinsic factor antibodies catches about 90% of cases, and a positive result is considered strong evidence of the condition.
Malabsorption Conditions
Anything that disrupts the part of the digestive tract where B12 is absorbed (primarily the end of the small intestine) can lead to deficiency. This includes Crohn’s disease, celiac disease, tropical sprue, and bacterial overgrowth in the small intestine. Surgical procedures that alter the gut, such as gastric bypass, gastrectomy, or removal of sections of the small intestine, also cut into B12 absorption. For people with permanent changes to their digestive anatomy, B12 replacement is typically needed for life.
Pancreatic insufficiency and certain parasitic infections can interfere with absorption as well, though these causes are less common in high-income countries.
Dietary Deficiency
B12 is found almost exclusively in animal products: meat, fish, eggs, and dairy. Strict vegans who don’t supplement are at clear risk of deficiency over time. Vegetarians who eat eggs or dairy generally get enough, though their levels may be borderline. Because the body’s B12 stores are large, it can take two to five years on a completely plant-based diet without supplements before deficiency develops.
Folate Deficiency
Folate (vitamin B9) deficiency tends to develop faster than B12 deficiency because the body’s folate reserves are much smaller, typically lasting only a few months. The causes fall into three broad categories: not eating enough, not absorbing enough, or needing more than usual.
Poor dietary intake is the most common trigger. Folate is found in leafy green vegetables, legumes, citrus fruits, and fortified grains. Diets low in fresh produce, combined with overcooking (which destroys folate), make deficiency more likely. Alcohol is a major contributor: heavy drinking both reduces folate absorption and increases how quickly the body uses it up. Chronic alcohol use is one of the most frequent causes of folate-related megaloblastic anemia in developed countries.
Pregnancy and breastfeeding sharply increase the body’s demand for folate because the nutrient is consumed in large quantities during rapid cell division in the developing fetus. Pregnant women are significantly more likely than non-pregnant women to become folate deficient, which is why prenatal supplementation is standard practice. Conditions involving rapid cell turnover, such as certain blood disorders where the body is constantly producing new red blood cells, also raise folate requirements.
Malabsorption from celiac disease, inflammatory bowel disease, or other conditions affecting the upper small intestine can also contribute, though this is less common than dietary causes.
Medications That Cause Megaloblastic Anemia
A number of drugs can trigger megaloblastic anemia by directly interfering with DNA synthesis or by disrupting how the body handles B12 and folate. This is sometimes called drug-induced megaloblastic anemia, and it can occur even when vitamin levels are technically adequate.
Methotrexate, used for autoimmune conditions and certain cancers, works by blocking folate metabolism. That’s actually how it slows disease, but the same mechanism can starve healthy cells of the folate they need to make DNA. Hydroxyurea, used in blood disorders like sickle cell disease, inhibits a different step in DNA building and commonly causes macrocytosis. Certain chemotherapy drugs, including 5-fluorouracil and 6-mercaptopurine, have similar effects.
Metformin, one of the most widely prescribed diabetes medications, can reduce B12 absorption over time. The effect is gradual, and not everyone on metformin becomes deficient, but B12 levels should be monitored in long-term users. Some anti-seizure medications and the antiviral drug zidovudine have also been linked to megaloblastic changes.
Telling B12 and Folate Deficiency Apart
Because B12 and folate deficiency produce nearly identical blood findings, distinguishing between them matters for treatment. Supplementing with folate alone can mask B12 deficiency on blood tests while neurological damage continues silently in the background.
The most reliable way to tell them apart is by measuring two metabolic markers. Methylmalonic acid (MMA) rises specifically in B12 deficiency. Homocysteine rises in both B12 and folate deficiency, and also in B6 deficiency and kidney disease, making it less specific. So if both MMA and homocysteine are elevated, B12 deficiency is the likely culprit. If homocysteine is high but MMA is normal, folate deficiency is more probable.
Serum B12 and folate levels are measured as a first step, but they don’t always tell the whole story. B12 blood levels can appear normal even when the body’s functional B12 is too low, which is why the metabolic markers are useful confirmation.
What Treatment Looks Like
Treatment depends on the underlying cause, but the core approach is straightforward: replace what’s missing.
For B12 deficiency caused by malabsorption or pernicious anemia, injections are the traditional starting point since the problem is that B12 can’t be absorbed through the gut. A typical course involves weekly injections for the first month, then monthly injections ongoing. High-dose oral B12 (1,000 to 2,000 micrograms daily) has been shown to work even in people with absorption problems, because at very high doses a small percentage of B12 passes directly through the intestinal wall without needing intrinsic factor. Some people prefer this to regular injections.
For dietary B12 deficiency in people with normal absorption, oral supplements at standard doses are effective. Folate deficiency is treated with oral folic acid supplements, along with dietary changes to include more folate-rich foods. In both cases, blood counts typically start improving within a week of beginning treatment, and the oversized red blood cells gradually return to normal size over two to three months.
When a medication is the cause, the options are to adjust the dose, switch to an alternative, or supplement with the affected vitamin while continuing the drug. Neurological symptoms from B12 deficiency, such as numbness, tingling, or difficulty with balance, may improve with treatment but can be permanent if the deficiency has been severe and prolonged.