Anemia happens when your body doesn’t have enough healthy red blood cells to carry oxygen to your tissues. The most common reason, by far, is not having enough iron, but the list of possible causes is longer than most people expect. It includes everything from heavy periods and poor dietary absorption to chronic inflammation, vitamin deficiencies, and inherited conditions. Understanding which type you have is the key to fixing it.
Iron Deficiency: The Most Common Cause
Iron deficiency accounts for the majority of anemia cases worldwide. Your body needs iron to build hemoglobin, the protein inside red blood cells that carries oxygen. When iron runs low, your red blood cells become smaller and paler, and they carry less oxygen per trip. The result is that familiar combination of fatigue, weakness, and shortness of breath.
There are three main ways iron deficiency develops: you’re not getting enough from food, you’re not absorbing what you eat, or you’re losing blood faster than you can replace the iron in it.
- Not enough dietary iron. This is the leading cause globally. Your body absorbs about 25% of the iron from animal sources like red meat, poultry, and fish, but 17% or less from plant sources like spinach, beans, and fortified grains. If you eat a plant-based diet, your overall iron bioavailability may be as low as 5% to 12%, compared to 14% to 18% for people who eat animal products regularly.
- Poor absorption. Even if your diet is rich in iron, conditions like celiac disease, inflammatory bowel disease, or a history of bariatric surgery can prevent your gut from absorbing it properly. Tea, coffee, red wine, and dairy products all contain compounds that inhibit iron absorption. Vitamin C does the opposite, helping your body pull more iron from food.
- Blood loss. Every milliliter of blood you lose takes iron with it. In premenopausal women, heavy menstrual periods are the most common culprit. In men and postmenopausal women, the source is more often the gastrointestinal tract: ulcers, polyps, inflammatory bowel disease, or in some cases, cancer. This is why doctors often investigate GI bleeding when someone develops unexplained iron deficiency.
Your body also burns through iron faster during certain life stages. Pregnancy, infancy, and adolescence all create surges in iron demand that can outpace what you’re taking in.
How Iron Deficiency Is Measured
A standard blood test measures your hemoglobin level and your ferritin, which reflects how much iron your body has stored. You’re generally considered anemic if your hemoglobin falls below 120 g/L for women or 130 g/L for men. But iron depletion starts well before your hemoglobin drops. Ferritin below 30 μg/L in adults now signals iron deficiency, a threshold that was raised from the older cutoff of 12 to 15 μg/L after evidence showed the previous standard missed too many cases. For pregnant women, treatment is recommended when ferritin drops below 50 μg/L.
Vitamin B12 and Folate Deficiency
Iron isn’t the only nutrient your body needs to make red blood cells. Vitamin B12 and folate are both essential for DNA synthesis during red blood cell production. When either one is low, your bone marrow produces red blood cells that are abnormally large and don’t function well. This is called megaloblastic anemia.
B12 deficiency is common in people over 50 (because stomach acid production declines with age, and you need acid to absorb B12), in vegans and strict vegetarians (since B12 is found almost exclusively in animal products), and in people with conditions that damage the stomach lining. Folate deficiency tends to show up in people with poor diets, heavy alcohol use, or malabsorption conditions. Pregnancy also increases folate demand significantly.
Chronic Disease and Inflammation
If you have a chronic condition like rheumatoid arthritis, kidney disease, diabetes, heart failure, or an ongoing infection, your anemia may not be about iron intake at all. It may be about how your body handles the iron it already has.
During inflammation, your immune system releases signaling molecules that ramp up production of a hormone called hepcidin. Hepcidin acts like a gatekeeper: it blocks iron from being absorbed in your gut and traps existing iron inside storage cells so it can’t be used to make new red blood cells. This is a defense mechanism. Your body is essentially hiding iron from invading bacteria, which also need iron to grow. The problem is that it starves your own red blood cell production in the process.
What makes this tricky is that your iron stores (ferritin) can look normal or even high on a blood test, because the iron is there, just locked away. Standard iron supplements often don’t help much because hepcidin blocks absorption of those too. Treating the underlying condition is usually the most effective path forward.
Inherited Conditions
Some people are anemic because of the way their hemoglobin is built. In sickle cell disease, a single change in the hemoglobin gene causes red blood cells to become rigid and crescent-shaped under low-oxygen conditions. These misshapen cells break apart easily, leading to chronic anemia, pain episodes, and organ damage over time. The repeated cycles of sickling and breaking make the cells fragile and short-lived.
Thalassemia is another inherited condition where the body produces less hemoglobin than normal, or hemoglobin that doesn’t work properly. The severity ranges widely. Some people carry a single gene variant and experience only mild anemia, while others need regular blood transfusions. Both sickle cell disease and thalassemia are more common in people with ancestry from Africa, the Mediterranean, the Middle East, and Southeast Asia, regions where these gene variants historically provided some protection against malaria.
Medications That Can Cause Anemia
Certain drugs can trigger your immune system to attack your own red blood cells, a process called drug-induced hemolytic anemia. The most common culprits include certain antibiotics (particularly cephalosporins and penicillin derivatives), some NSAIDs, and a few less common medications like methyldopa and nitrofurantoin. This type of anemia typically resolves after the medication is stopped, though it sometimes takes weeks for red blood cell counts to recover.
Other medications cause anemia more indirectly. Long-term use of aspirin or other anti-inflammatory drugs can cause slow, chronic bleeding from the stomach lining. Proton pump inhibitors for acid reflux can reduce iron and B12 absorption over time.
How Different Types Feel
Most anemia develops gradually, so symptoms creep up slowly enough that you adjust without realizing it. The classic signs, fatigue, weakness, pale skin, and feeling winded during normal activities, are shared across nearly all types. But there are some patterns worth knowing.
Iron deficiency specifically can cause cravings for ice, dirt, or starch (a phenomenon called pica), brittle nails, cold hands and feet, and a sore or swollen tongue. B12 deficiency often adds neurological symptoms: tingling or numbness in your hands and feet, difficulty with balance, brain fog, and sometimes mood changes. Hemolytic anemias, where red blood cells are being destroyed faster than they’re made, can cause jaundice (yellowing of the skin and eyes) and dark urine.
When anemia develops suddenly from acute blood loss, the symptoms are more dramatic: dizziness, rapid heartbeat, lightheadedness, and chest pain. Chronic anemia that worsens gradually can eventually produce the same symptoms as it becomes more severe.
Why the Cause Matters More Than the Label
Taking iron supplements is the right answer for some people and completely ineffective for others. If your anemia comes from B12 deficiency, iron won’t help. If it comes from chronic inflammation, your body will block the iron before it reaches your bloodstream. If it comes from undiagnosed celiac disease, no amount of oral iron will be absorbed properly until the underlying gut damage is addressed.
This is why a blood test that only shows low hemoglobin isn’t enough. A complete workup typically includes ferritin, B12, folate, and a look at the size and shape of your red blood cells. Small, pale cells point toward iron deficiency or thalassemia. Large cells suggest B12 or folate problems. Normal-sized cells with low hemoglobin can indicate chronic disease, early deficiency, or a combination of causes. In men and postmenopausal women with unexplained iron deficiency, screening for GI bleeding is standard because it can be the first sign of something more serious in the digestive tract.