Your blood can thicken when it has too many red blood cells, too many clotting proteins, or too little fluid. Sometimes the cause is as simple as dehydration; other times it signals a genetic condition, a medication side effect, or an underlying disease. The result is the same: blood that moves sluggishly through your vessels and is more likely to form dangerous clots.
Dehydration: The Most Common Cause
The simplest reason blood thickens is not having enough water in it. Your blood is roughly half liquid plasma, and when that plasma volume drops, the concentration of red blood cells and proteins rises. In one study, subjects who sat for four hours in a dry environment without drinking lost about 110 milliliters of fluid per hour. After just two hours, their plasma volume had fallen by 3.4%, and their blood viscosity had jumped by 9.3%. That’s a meaningful change from something most people wouldn’t even notice happening.
This is why blood clot risk rises during long flights, extended bed rest, or any situation where you’re not drinking enough. The fix is straightforward, but the speed at which it happens catches people off guard. You don’t need to be severely dehydrated to see the effect.
Too Many Red Blood Cells
Red blood cells make up the largest solid component of your blood, and when their numbers climb too high, blood becomes measurably thicker. The proportion of your blood occupied by red blood cells is called your hematocrit. Normal hematocrit runs 40 to 54% for men and 36 to 48% for women.
When hematocrit pushes above 49% in men or 48% in women, doctors start considering a condition called polycythemia. This can happen because your bone marrow is overproducing cells on its own (polycythemia vera, a blood cancer) or because something is driving the overproduction, like chronic low oxygen from lung disease, sleep apnea, or living at high altitude. Either way, the thicker blood raises the risk of both arterial and venous clots, which can lead to heart attack or stroke.
Inherited Clotting Disorders
Some people are born with blood that clots more easily than it should. The most common genetic cause is a mutation called Factor V Leiden, which makes one of the body’s natural braking systems for clotting less effective. Between 3 and 8 percent of people with European ancestry carry one copy of this mutation. Normally, about 1 in 1,000 people develop an abnormal blood clot in any given year. Carrying one copy of Factor V Leiden raises that risk to 3 to 8 in 1,000. Carrying two copies pushes it as high as 80 in 1,000.
The second most common inherited risk factor is a prothrombin gene mutation, which causes your body to produce higher levels of prothrombin, a protein that helps form clots. Other inherited conditions include deficiencies in the proteins that normally keep clotting in check: protein C, protein S, and antithrombin. There’s also a condition called sticky platelet syndrome, where platelets overreact to normal chemical signals and clump together too readily.
Many people with these mutations never know they have them until a clot appears, often triggered by a second risk factor like surgery, pregnancy, or a long period of immobility.
Hormones and Medications
Estrogen-based medications are one of the better-known triggers for thicker, clot-prone blood. Oral estrogen, whether in birth control pills or hormone replacement therapy, passes through the liver and stimulates production of several clotting proteins, including factors VII, VIII, and IX. For postmenopausal women, oral estrogen raises the risk of a venous blood clot roughly two to five times compared to nonusers. One large study found the odds ratio for clots was 4.2 for oral estrogen users versus just 0.9 for women using estrogen patches, which bypass the liver entirely.
The type of progesterone matters too. Natural progesterone does not appear to increase clot risk, but synthetic versions like medroxyprogesterone acetate do. If you’re considering hormone therapy and have other clotting risk factors, the delivery method and formulation can make a real difference.
Smoking and Platelet Stickiness
Smoking thickens blood through several routes at once. It raises levels of fibrinogen, the protein that forms the structural mesh of a blood clot, and increases production of clotting factor VII. On top of that, it makes platelets stickier and quicker to aggregate. Research shows that platelet activation markers are significantly higher in smokers than nonsmokers, and the lag time before platelets clump together in response to triggers is shorter. The combined effect is blood that’s both thicker and faster to form clots, which is a major reason smoking dramatically increases heart attack and stroke risk even in otherwise healthy people.
Cancer and Blood Clots
Cancer is the second most common acquired cause of abnormal blood thickening. Tumors, particularly mucin-producing cancers of the pancreas, lung, stomach, and ovary, release substances that activate the clotting system directly. The primary culprit is tissue factor, a protein normally confined to the outside of blood vessels that triggers clot formation when it contacts blood. Tumor cells expose tissue factor on their surfaces and shed tiny particles loaded with it into the bloodstream. Some cancers also produce a separate enzyme that directly activates clotting factor X, essentially short-circuiting the normal clotting process.
This connection between cancer and clotting has been recognized since 1865, and an unexplained blood clot is sometimes the first sign that leads to a cancer diagnosis. Low oxygen levels within tumors can also ramp up genes that promote clotting, including those that suppress the body’s clot-dissolving system.
Viral Infections and Inflammation
COVID-19 brought widespread attention to how infections can thicken blood. Hospitalized COVID patients showed D-dimer levels (a marker of active clot formation) far above the normal ceiling of 0.45 micrograms per milliliter. The median D-dimer on admission was 2.35, more than five times normal. Patients who deteriorated enough to need a ventilator had median levels of 3.79 on admission and peak levels of 7.58. Among those who died, peak D-dimer averaged 5.64, compared to 2.29 in survivors.
While COVID was extreme, many serious infections and inflammatory conditions can push the clotting system into overdrive. The body’s inflammatory response and its clotting response share overlapping triggers, so widespread inflammation from any cause can shift blood toward a more clot-prone state.
Autoimmune Conditions
The most common acquired clotting disorder is antiphospholipid syndrome, an autoimmune condition where the immune system produces antibodies against components of cell membranes called phospholipids. These antibodies interfere with the normal regulation of clotting and can cause recurrent blood clots in both veins and arteries. Antiphospholipid syndrome can occur on its own or alongside other autoimmune diseases like lupus, and it’s a significant cause of pregnancy complications including recurrent miscarriage.
When the Body Can’t Dissolve Clots
Blood thickness isn’t only about how fast clots form. It also depends on how well your body breaks them down. A healthy clotting system constantly balances clot formation with clot dissolution. When the dissolving side fails, even normal clot formation leads to buildup. This can happen with a deficiency in plasminogen (the protein that gets converted into the clot-dissolving enzyme), an increase in plasminogen activator inhibitor (which blocks the dissolving process), or abnormal fibrinogen that resists being broken apart. These conditions are less well known than the ones that speed up clotting, but the end result is similar: clots that persist and grow instead of being cleared away.
What Thick Blood Does to Your Body
When blood viscosity stays elevated, the consequences center on reduced flow and increased clotting. In mild cases, you might notice headaches, dizziness, or blurred vision as blood struggles to move efficiently through small vessels. More severe hyperviscosity can cause neurological symptoms including confusion, hearing loss, seizures, and stroke. Heart failure can develop as the heart works harder to pump thicker blood. Mucosal bleeding, particularly nosebleeds and bleeding gums, is another hallmark, because the sluggish blood flow damages small vessels near the surface.
The combination of slow flow and overactive clotting proteins creates conditions ripe for deep vein thrombosis in the legs, which can break loose and travel to the lungs as a pulmonary embolism. In the most serious cases, widespread clotting can starve multiple organs of blood supply simultaneously.