Untreated anemia forces your body to compensate for carrying less oxygen in the blood, and over time those compensations cause real damage. The consequences range from persistent fatigue and poor exercise tolerance to heart problems, cognitive decline, weakened immunity, and a measurably higher risk of death. How severe the consequences get depends on how low your hemoglobin drops, how long it stays there, and your age and overall health.
For reference, anemia in adults is defined as hemoglobin below 120 g/L for women and below 130 g/L for men. Moderate anemia falls in the 80 to 109 g/L range, and severe anemia is anything below 80 g/L. The lower you go and the longer you stay there, the more serious the fallout.
Your Heart Works Harder Until It Can’t
When your blood carries less oxygen, your heart compensates by pumping faster and harder to deliver the same amount to your tissues. In the short term, this feels like a racing pulse or breathlessness during activities that used to be easy. Over months or years, the extra workload causes the heart muscle to thicken, particularly the left ventricle, the chamber responsible for pumping blood to the rest of your body. This thickening, called left ventricular hypertrophy, makes the heart stiffer and less efficient.
Eventually, a heart that’s been overworking can’t keep up. Fluid backs up into the lungs and legs, and you develop heart failure. People who already have heart disease are especially vulnerable: anemia accelerates the cycle of worsening cardiac function, and the combination of the two conditions is significantly harder to manage than either one alone.
Higher Risk of Death, Especially Under 75
A large study tracking participants in the Atherosclerosis Risk in Communities cohort found that people with anemia had a 37% higher risk of dying from any cause compared to those with normal hemoglobin, even after adjusting for other health conditions. The risk was particularly pronounced for cancer mortality, which was 46% higher in anemic participants.
Age made a notable difference. For people under 75, anemia was associated with a 73% increase in all-cause mortality. For those 75 and older, the increase was 27%. Women with anemia faced a 53% higher mortality risk, while men had a 24% increase. These numbers reflect the compounding effect of chronic oxygen deprivation on multiple organ systems over time.
Cognitive Decline and Dementia Risk
About 17% of older adults have anemia, and for this group the brain is one of the most vulnerable organs. A UK Biobank study followed more than 207,000 dementia-free adults aged 60 and older for up to 16 years. Those with anemia had a 57% higher risk of developing dementia compared to those without it, and dementia onset arrived roughly a year and a half earlier on average.
The connection makes biological sense: the brain consumes a disproportionate share of the body’s oxygen, so it’s one of the first organs to suffer when oxygen delivery drops. Chronic anemia also contributes to frailty, a state of increased vulnerability linked to mitochondrial dysfunction and poor cognition. Beyond dementia, anemia in older adults is tied to higher rates of falls, disability, and reduced quality of life.
Your Immune System Weakens
Iron is essential for immune cell function, and iron deficiency anemia (the most common type) directly impairs your body’s ability to fight infections. Without adequate iron, T cells, the white blood cells responsible for targeting infected or abnormal cells, don’t mature properly. Their ability to multiply and activate in response to threats is reduced, and they produce fewer of the signaling molecules that coordinate the immune response.
B cells, which produce antibodies, are similarly affected. Animal and human studies have found that iron deficiency lowers levels of key inflammatory signals like interferon-gamma, interleukin-6, and TNF-alpha. These disruptions worsen as anemia becomes more severe, leaving you increasingly susceptible to bacterial infections. Severe anemia has direct, profound effects on the cellular immune response to bacteria in the bloodstream.
Reduced Physical Capacity
Anemia limits how much oxygen your muscles can use during exercise, which directly reduces your aerobic capacity. Even mild anemia has a measurable impact. A study of over 3,600 military men found that those with mild anemia were 47% more likely to rank in the worst 10% of performers on a 3,000-meter run compared to non-anemic peers, after accounting for fitness habits, body size, and other health factors.
The effect is specific to endurance activities. The same study found no reduction in performance on sit-ups and actually found mildly anemic men performed slightly better on push-ups, suggesting that short bursts of anaerobic effort aren’t affected in the same way. But for anything requiring sustained oxygen delivery (walking uphill, climbing stairs, running, cycling) untreated anemia creates a ceiling on what your body can do. The longer it persists, the more deconditioned you become, creating a cycle of fatigue and inactivity that’s hard to break.
Serious Risks During Pregnancy
Untreated iron deficiency anemia during pregnancy raises the risk of preterm birth, low birth weight, and dangerous bleeding after delivery. The stakes are not evenly distributed: Black women in the United States experience higher rates of iron deficiency anemia during pregnancy and are less likely to be screened for it by their healthcare providers. This screening gap leads to delayed diagnosis and treatment, compounding already existing disparities. Pregnant Black women face up to a 5-fold higher risk of death from postpartum hemorrhage and a 2-fold higher risk of needing blood transfusions compared to other groups.
For the developing baby, the mother’s iron status directly affects growth and oxygen supply. Chronic oxygen deprivation in utero contributes to restricted fetal development and increases the likelihood of complications during delivery.
Lasting Effects on Children’s Development
When iron deficiency anemia occurs in infancy or early childhood, the damage can extend well beyond the bloodstream. A systematic review published in Balkan Medical Journal found that children with a history of early iron deficiency showed lower cognitive function when tested at ages 4, 8 to 9, and 10 years old, compared to children who were never iron deficient. Four studies evaluating motor skills found the same pattern of impairment, and seven out of nine studies examining behavior reported significantly more behavioral problems in the iron-deficient group.
What makes this particularly concerning is that some of these deficits persist even after iron levels are corrected. The developing brain is especially sensitive to iron deprivation during critical windows of growth in the first two years of life. Neurons need iron to form the protective coating around nerve fibers that allows signals to travel quickly and efficiently. When that process is disrupted, the effects on learning, coordination, and emotional regulation can be long-lasting.
The Kidney Connection
Anemia and kidney disease have a two-way relationship that can spiral if left unchecked. Healthy kidneys produce a hormone that signals your bone marrow to make red blood cells. As kidney function declines, less of this hormone is produced, and anemia develops. But anemia itself stresses the kidneys by reducing oxygen delivery to kidney tissue, which can accelerate the loss of function. According to the National Institute of Diabetes and Digestive and Kidney Diseases, anemia is uncommon in early kidney disease but becomes increasingly prevalent as more kidney function is lost. Without treatment, each condition worsens the other.