Macrocytic anemia is when your red blood cells are larger than normal and you don't have enough of them

Macrocytic anemia means your bone marrow is making red blood cells that are bigger than they should be, and you have fewer of them circulating in your blood. The word "macrocytic" comes from "macro" (large) and "cytic" (cell). A doctor measures this using a test called the MCV, or mean corpuscular volume, which shows the average size of your red blood cells. When that number is high, your cells are oversized.

The problem is that these large cells don't carry oxygen as efficiently as normal-sized ones do. Your body needs a certain number of red blood cells to deliver oxygen to your organs and tissues. When you have fewer cells, and the ones you have are working less efficiently, you feel tired, short of breath, or weak. The underlying cause matters because it determines what your doctor will recommend.

Macrocytic anemia is not a diagnosis by itself—it's a sign that something else is going on. The two main categories are megaloblastic (caused by problems making DNA in your cells) and non-megaloblastic (caused by other factors). Knowing which type you have points toward the actual problem.

Key Takeaways

  • Macrocytic anemia means your red blood cells are larger than normal and your body doesn't have enough of them to carry oxygen properly.
  • The MCV blood test measures the average size of your red blood cells and is how doctors detect macrocytic anemia.
  • Vitamin B12 deficiency and folate deficiency are the most common causes and are often reversible with supplementation.
  • Symptoms include fatigue, shortness of breath, weakness, and sometimes tingling in your hands or feet, depending on the cause.
  • Your doctor will order additional blood tests to find the underlying cause before recommending treatment.

Megaloblastic macrocytic anemia: B12 and folate deficiency

The most common form of macrocytic anemia is megaloblastic, which happens when your body cannot make DNA properly. This usually comes from not having enough vitamin B12 or folate (also called folic acid). Both vitamins are essential for your cells to divide and mature correctly. Without them, your bone marrow produces fewer, larger, and less functional red blood cells.

B12 deficiency can develop for several reasons. Some people don't eat enough animal products (meat, fish, eggs, dairy), which are the main dietary sources. Others have a condition called pernicious anemia, where the stomach lining doesn't produce a protein called intrinsic factor that your body needs to absorb B12 from food. Certain medications, like metformin for diabetes or proton pump inhibitors for acid reflux, can also lower B12 levels over time. Some people have had stomach surgery that removes the part of the intestine where B12 is absorbed.

Folate deficiency is usually dietary. It comes from not eating enough leafy greens, legumes, asparagus, or fortified grains. Alcohol use disorder can prevent your body from absorbing and storing folate. Certain medications, including some seizure drugs and methotrexate for autoimmune conditions, interfere with folate metabolism.

Non-megaloblastic macrocytic anemia: Other causes

Non-megaloblastic macrocytic anemia means your red blood cells are large but your bone marrow is not having trouble making DNA. This category includes several different problems. Alcohol use disorder is a common cause—it damages bone marrow directly and also leads to poor nutrition. Liver disease can cause macrocytic anemia because the liver plays a role in red blood cell maturation. Hypothyroidism (an underactive thyroid) slows down cell production and can result in larger cells.

Some medications cause this type of anemia. Chemotherapy drugs, antiretroviral drugs for HIV, and some antibiotics can damage bone marrow or interfere with cell production. Certain blood disorders, like myelodysplastic syndromes, cause the bone marrow to produce abnormal cells. Hemolytic anemia—where red blood cells break down faster than normal—can trigger the bone marrow to release immature, larger cells in an attempt to keep up with demand.

Reticulocytosis is when your bone marrow is working overtime and releasing young red blood cells (reticulocytes) into your bloodstream before they fully mature. These young cells are larger than mature ones. This can happen after blood loss, during recovery from anemia, or in response to certain medications or conditions that stimulate red blood cell production.

Symptoms you might notice

The symptoms of macrocytic anemia overlap with other types of anemia because they all come down to your body not having enough oxygen. You might feel unusually tired even after sleeping, get short of breath during normal activities like climbing stairs, or feel weak or dizzy. Some people notice they can't concentrate as well or feel foggy mentally.

If your macrocytic anemia is caused by B12 deficiency, you may also experience peripheral neuropathy—tingling, numbness, or a pins-and-needles sensation in your hands and feet. This happens because B12 is essential for nerve function. If the deficiency goes untreated for a long time, these nerve symptoms can become permanent. You might also notice your tongue feels swollen or smooth, or you have mouth sores.

Symptoms develop gradually in most cases, so you might not notice them at first. Some people feel fine until a blood test catches the anemia during a routine checkup. The severity depends on how low your red blood cell count is and how quickly it dropped.

How doctors diagnose macrocytic anemia

Your doctor starts with a complete blood count (CBC), which measures your red blood cell count, hemoglobin level, and MCV. If your MCV is above the normal range (usually above 100 femtoliters, though this varies slightly by lab), you have macrocytic anemia. The CBC also shows whether your red blood cells look normal under a microscope or have an unusual shape.

To find the cause, your doctor will order a peripheral blood smear, where a lab technician looks at your blood cells under a microscope and describes what they see. They will also check your B12 and folate levels with a blood test. If B12 is low, further testing might include a test for intrinsic factor antibodies (to check for pernicious anemia) or a methylmalonic acid test (which rises when B12 is deficient).

Depending on what those results show, your doctor may order additional tests. A reticulocyte count shows how many young red blood cells your bone marrow is producing. Liver function tests check whether liver disease is involved. If your doctor suspects a bone marrow problem, they might recommend a bone marrow biopsy, though this is not routine.

Treatment depends on the underlying cause

If you have B12 deficiency, treatment is straightforward: B12 supplementation. This can be given as injections (usually monthly), oral tablets, nasal spray, or sublingual (under-the-tongue) tablets. Injections bypass the digestive system entirely, so they work even if your body can't absorb B12 from food. If your deficiency is dietary, oral supplements or eating more B12-rich foods may be enough. If you have pernicious anemia, you will need injections long-term because your body cannot absorb B12 naturally.

Folate deficiency is treated with folate supplementation, usually as oral tablets. Dietary changes—eating more leafy greens, legumes, and fortified grains—can help prevent it from returning. If alcohol use is the underlying problem, addressing that is essential because folate supplements alone won't work if you continue drinking heavily.

For non-megaloblastic causes, treatment targets the underlying condition. If liver disease is responsible, managing the liver disease may improve the anemia. If a medication is causing it, your doctor might switch you to a different drug. If hypothyroidism is the cause, thyroid hormone replacement usually resolves the anemia. Some causes, like myelodysplastic syndromes, require more complex management and may involve blood transfusions or other treatments.

What happens if macrocytic anemia goes untreated

Untreated macrocytic anemia gets worse over time. Your red blood cell count drops further, oxygen delivery to your organs declines, and your symptoms intensify. You might become so fatigued that daily activities become difficult. Your heart has to work harder to pump blood and deliver oxygen, which can strain your cardiovascular system.

If the cause is B12 deficiency, nerve damage from prolonged deficiency can become permanent. Tingling and numbness may progress to difficulty walking or loss of coordination. Cognitive changes like memory problems or confusion can develop. These neurological complications are why it matters to catch and treat B12 deficiency relatively early.

The good news is that most causes of macrocytic anemia respond well to treatment once the underlying problem is identified. B12 and folate deficiencies are reversible. Even when the cause is more complex, knowing what it is allows your doctor to manage it effectively and prevent complications.

Frequently Asked Questions

Can macrocytic anemia go away on its own?

It depends on the cause. If your macrocytic anemia is from a temporary situation like heavy alcohol use that you stop, it may improve as your body recovers. But if it's from B12 deficiency, folate deficiency, or a chronic condition like liver disease, it will not resolve without treatment. Your doctor needs to identify the cause first.

Is macrocytic anemia serious?

Macrocytic anemia itself is a sign that something needs attention, but it's not immediately life-threatening in most cases. However, if it's caused by B12 deficiency and goes untreated for months or years, nerve damage can become permanent. Severe anemia of any type can strain your heart. Early diagnosis and treatment prevent these complications.

What's the difference between macrocytic and microcytic anemia?

Macrocytic anemia means your red blood cells are too large; microcytic anemia means they are too small. Microcytic anemia is usually caused by iron deficiency or problems with hemoglobin production. The two types have different causes and require different treatments, which is why the MCV measurement matters.

Can I treat macrocytic anemia with diet alone?

If your macrocytic anemia is from folate deficiency and you catch it early, dietary changes may help. But if it's from B12 deficiency, especially if you have pernicious anemia, diet alone won't work—you need supplementation or injections because your body cannot absorb B12 properly. Your doctor will tell you what approach is right for your situation.

Will B12 injections give me energy right away?

Most people start feeling better within days to a week of starting B12 treatment, though it can take several weeks for fatigue to fully resolve. Nerve symptoms take longer—sometimes weeks or months—to improve, and if the deficiency was severe or long-standing, some nerve damage may not reverse completely.