“Low blood” typically refers to one of two things: a low blood count (anemia, meaning too few red blood cells or too little hemoglobin) or low blood pressure (hypotension). Both can cause fatigue, dizziness, and feeling faint, which is why they’re easy to confuse. The causes are quite different, though, so this article covers both.
Low Blood Count: What Anemia Actually Means
Anemia is the most common meaning of “low blood.” It means your blood doesn’t carry enough oxygen because you have too few red blood cells or not enough hemoglobin, the protein inside those cells that binds oxygen. For men, hemoglobin below 13 g/dL is considered low. For non-pregnant women, the cutoff is 12 g/dL. During pregnancy, hemoglobin below 11 g/dL qualifies as anemia.
The severity matters. Mild anemia (hemoglobin around 11 to 12.9 for men, 11 to 11.9 for women) may cause only subtle tiredness. Moderate anemia, with levels between 8 and 10.9, often brings noticeable fatigue, shortness of breath during activity, and pale skin. Severe anemia, below 8 g/dL, can cause a racing heartbeat, chest pain, and dangerously low oxygen delivery to your organs.
Iron Deficiency: The Most Common Cause
Iron is essential for making hemoglobin. Without enough iron, your body produces smaller, paler red blood cells that carry less oxygen. Three main pathways lead to iron deficiency: not getting enough iron from food, not absorbing it properly, or losing blood.
Blood loss is the leading cause in adults. Heavy menstrual periods are the most common source in premenopausal women. In older adults and men, slow bleeding from the digestive tract (ulcers, polyps, or colon problems) is a frequent culprit. Pregnancy increases iron needs substantially because the body has to supply blood for both the mother and the growing baby.
Absorption problems also play a role. Conditions like celiac disease damage the lining of the small intestine, making it harder for your body to pull iron from food. Stomach surgery, including weight-loss procedures, can have the same effect. Even certain common medications, like proton pump inhibitors used for acid reflux, can reduce iron absorption over time.
Vitamin Deficiencies Beyond Iron
Your body also needs vitamin B12 and folate to produce healthy red blood cells. When either is lacking, the bone marrow produces abnormally large, poorly functioning red blood cells. This is called megaloblastic anemia.
B12 deficiency can come from a diet very low in animal products (since B12 is found mainly in meat, fish, eggs, and dairy), but it’s more often caused by absorption problems. Stomach inflammation, H. pylori infection, and surgical removal of parts of the stomach or small intestine all interfere with B12 uptake. Metformin, a widely used diabetes medication, can also lower B12 levels over years of use. Even prolonged use of nitrous oxide (used recreationally or during dental procedures) inactivates B12 in the body.
Folate deficiency tends to stem from low dietary intake of leafy greens, beans, and fortified grains. Pregnancy sharply increases folate needs, which is why prenatal vitamins include it. Alcohol use also depletes folate stores.
Chronic Kidney Disease and Blood Production
Your kidneys do more than filter waste. They produce a hormone that signals your bone marrow to make red blood cells. As kidney function declines, production of this hormone drops, and red blood cell output falls with it. By the time kidney disease reaches its most advanced stage, nearly all patients are anemic.
The problem compounds itself. Chronic kidney disease also triggers ongoing inflammation, which blocks iron from being released from your body’s storage sites and into the bloodstream where it’s needed. On top of that, a protein called hepcidin builds up as kidney filtration worsens. Hepcidin reduces iron absorption from food and locks iron inside cells, starving the bone marrow of a key ingredient. Red blood cells in people with advanced kidney disease also have a shorter lifespan than normal, meaning they break down faster than the body can replace them.
Low White Blood Cells and Low Platelets
“Low blood” can also refer to low white blood cells (leukopenia) or low platelets (thrombocytopenia), especially if a blood test flagged one of these.
White blood cells fight infection. Their levels commonly drop during viral infections, which is usually temporary: counts bounce back once the illness passes. Autoimmune diseases like lupus and rheumatoid arthritis can chronically lower white cells because the immune system attacks its own blood components. Cancer treatments, particularly chemotherapy, are another major cause. Bone marrow disorders like aplastic anemia and multiple myeloma directly impair the marrow’s ability to produce white cells. Nutritional deficiencies, including B12 and folate, can also contribute.
Platelets are the cells that help your blood clot. When a viral infection triggers widespread inflammation, platelets get consumed faster than usual and may also be pulled out of circulation by the spleen. Mild platelet drops combined with low white cells in someone who feels sick often point to a viral cause. Certain medications, immune disorders, and bone marrow problems can lower platelets as well.
Low Blood Pressure: A Different Problem
Low blood pressure, or hypotension, is the other common interpretation of “low blood.” It means the force of blood pushing against your artery walls is lower than usual. While some people naturally run low without problems, a significant drop can cause dizziness, blurred vision, nausea, and fainting.
Dehydration is one of the most frequent triggers. When your body loses more fluid than it takes in (from vomiting, diarrhea, excessive sweating, or simply not drinking enough), blood volume drops and pressure falls. Long periods of bed rest can also decondition the cardiovascular system, making blood pressure unstable when you finally stand up.
Pregnancy commonly lowers blood pressure, especially in the first 24 weeks. Blood vessels expand rapidly to accommodate the growing blood supply, and pressure drops as a result. This usually resolves later in pregnancy or after delivery.
Medications That Lower Blood Pressure
Several classes of medication can cause blood pressure to drop too low, particularly when standing up quickly. Heart and blood pressure medications are the most obvious culprits: diuretics (water pills), beta-blockers, calcium channel blockers, and nitrates all lower pressure by design and can overshoot. Alpha-blockers, sometimes prescribed for prostate problems, are another common cause.
Less obviously, many psychiatric medications also lower blood pressure. Antidepressants, antipsychotics, trazodone (a sleep aid), and benzodiazepines (anti-anxiety drugs) are all linked to drops in blood pressure upon standing. Opioid pain medications and levodopa, used for Parkinson’s disease, carry the same risk. Older adults taking multiple medications face the highest risk because the effects stack.
Nervous System Causes
Your nervous system constantly adjusts blood pressure to match what your body needs. When communication between the heart and brain misfires, blood pressure can plummet. One version, called neurally mediated hypotension, mostly affects young adults and children. It typically happens after standing in one place for a long time, like waiting in line or standing during an assembly.
Older adults with Parkinson’s disease or other conditions affecting the autonomic nervous system (the part that controls breathing, heart rate, and digestion without conscious effort) are especially vulnerable to sudden blood pressure drops. A rare condition called multiple system atrophy combines progressive nervous system damage with chronic low blood pressure on standing.
How to Tell Which Type You Have
The symptoms overlap considerably. Fatigue, dizziness, lightheadedness, and feeling like you might pass out can come from low blood counts or low blood pressure. A few clues help distinguish them. Low blood pressure symptoms tend to be worst when you stand up suddenly and improve when you sit or lie down. Anemia symptoms are more constant: persistent tiredness, pale skin (check the inside of your lower eyelids and nail beds), shortness of breath during normal activities, and cold hands and feet.
A simple blood test called a complete blood count (CBC) reveals whether your red blood cells, white blood cells, or platelets are low. Blood pressure is measured with a cuff in seconds. If you’re experiencing persistent fatigue or dizziness, both tests are typically part of an initial workup and can quickly narrow down the cause.