Venesection is a medical procedure in which a controlled amount of blood is drawn from a vein to treat conditions caused by too much iron or too many red blood cells in the body. You may also hear it called therapeutic phlebotomy or, historically, bloodletting. While it sounds simple, it is a targeted treatment with specific goals: lowering iron stores, reducing blood thickness, or both. It is currently approved for three main conditions: hemochromatosis, polycythemia vera, and porphyria cutanea tarda, though it is also used in several other situations.
How Venesection Works
Each unit of blood (about 500 mL, roughly a pint) contains between 200 and 250 mg of iron, depending on your hemoglobin level. When that blood is removed, your body has to dip into its iron reserves to make new red blood cells. Over repeated sessions, this steadily drains excess iron from the liver, heart, and other tissues where it may be causing damage.
The benefits go beyond simple iron math. When iron levels in the blood get too high, free-floating iron particles generate reactive oxygen species, which are molecules that damage cells. Venesection appears to reduce these harmful particles relatively quickly, which may explain why liver function often improves in patients before their overall iron stores have fully normalized. Removing blood also lowers hepcidin, a hormone that controls how iron moves around the body. When hepcidin drops, iron becomes more available for healthy red blood cell production, which is why hemoglobin levels can actually rise over a course of treatment even though blood is being taken away.
In conditions where the problem is too many red blood cells rather than too much iron, venesection works by directly thinning the blood. A high concentration of red blood cells makes blood thicker and more likely to clot, so bringing that concentration down reduces the risk of stroke and other clotting events.
Conditions Treated With Venesection
Hemochromatosis
Hemochromatosis is the most common reason for venesection. In this inherited condition, the body absorbs too much iron from food and has no efficient way to get rid of it. Iron accumulates in the liver, heart, and pancreas, eventually causing organ damage if left untreated. Venesection is the standard first-line treatment. For men and postmenopausal women, treatment typically begins when serum ferritin (a blood marker of iron stores) rises above 300 µg/L. For premenopausal women, the threshold is lower, around 200 µg/L.
Polycythemia Vera
Polycythemia vera is a blood cancer in which the bone marrow produces too many red blood cells. The resulting blood thickness raises the risk of dangerous clots. The treatment target is clear: keep the hematocrit (the percentage of blood volume occupied by red blood cells) below 45%. This target comes from a major clinical trial called CYTO-PV, which showed that maintaining hematocrit below this level significantly reduced the rate of cardiovascular events and death. If a patient develops an acute blood clot, venesection is performed urgently to bring the hematocrit under control.
Porphyria Cutanea Tarda
This condition causes painful blistering of the skin, particularly on sun-exposed areas like the hands and face. It is linked to iron overload in the liver, which interferes with the production of heme, a component of hemoglobin. Reducing iron through venesection helps normalize heme production and resolve skin symptoms.
Other Uses
Venesection is also used in several less common scenarios. Patients with chronic lung disease or cyanotic congenital heart disease sometimes develop dangerously high red blood cell counts as the body overcompensates for low oxygen levels. Guidelines recommend venesection for these patients when the hematocrit exceeds 56%, with a target of bringing it down to 50 to 52%. Kidney transplant recipients who develop high red blood cell counts after surgery may also be treated with venesection, typically targeting a hematocrit below 45%. Some patients with sickle cell disease benefit from venesection as well, either alone or alongside other treatments.
What Happens During the Procedure
Venesection looks and feels a lot like donating blood. A needle is inserted into a vein, usually in the arm, and blood flows into a collection bag. A standard session removes one unit (500 mL). For people with a smaller body size, anemia, or heart or lung problems, a half unit (250 mL) is taken instead. The actual blood collection typically takes less than 10 minutes, though the full appointment is longer once you factor in preparation and a short rest period afterward.
How often you need venesection depends on the condition being treated and how your body responds. During the initial phase of hemochromatosis treatment, sessions may happen weekly or every two weeks until iron stores reach the target range. After that, maintenance sessions might be needed only every few months. For polycythemia vera, the frequency depends on how quickly your hematocrit climbs back up between sessions.
Side Effects and Recovery
Most people tolerate venesection well, but side effects are possible. The most common ones are lightheadedness, dizziness, and discomfort at the needle site. Bruising, minor swelling, or a small amount of bleeding where the needle went in can also occur. If you feel dizzy afterward, sitting down and placing your head between your knees helps, or lying flat with your feet slightly elevated on a couple of pillows.
Recovery is straightforward. You should leave the pressure bandage on the needle site for 3 to 6 hours. If there is swelling or bruising, applying an ice pack in 10 to 15 minute intervals can help. Drink at least 8 to 10 glasses of water over the rest of the day to replace lost fluid. Avoid alcohol, caffeine, hot baths, and strenuous exercise for the remainder of the day. If you smoke, wait at least an hour after the procedure. Most people return to normal activities the following day.
Venesection vs. Blood Donation
The physical process is nearly identical, but the purpose and context are different. Blood donation is voluntary and the collected blood goes to other patients. Venesection is a prescribed medical treatment for a diagnosed condition, and the blood removed is typically discarded rather than used for transfusion, since it may come from someone with a condition that makes their blood unsuitable for others. The volume removed per session is similar (around 500 mL), but venesection schedules are driven by lab results and clinical targets rather than donation center intervals. In some countries, people with hemochromatosis whose blood is otherwise healthy are allowed to donate through standard blood services, which serves a dual purpose.
Long-Term Outlook
For many patients, venesection is not a one-time fix but an ongoing part of managing a chronic condition. In hemochromatosis, once iron levels normalize, periodic maintenance sessions keep them in check, sometimes for life. In polycythemia vera, venesection is often combined with medication to control red blood cell production over the long term. The decline in iron stores during treatment is not always steady. Some patients experience a plateau partway through, followed by a second decline as iron is mobilized from deep tissue stores in organs like the liver and heart. This is normal and does not mean the treatment has stopped working.
Despite its ancient origins, venesection remains one of the most effective treatments available for iron overload and red blood cell excess. It is low-tech, low-risk, and for many people, the single most important intervention in preventing serious organ damage.