context: Polycythemia vera ( PV) is a rare blood cancer that causes your body to make too many red blood cells. Extra cells may not sound like a problem, but they are. They thicken your blood, which means it doesn't flow as quickly, so it's more like maple syrup than water. When your blood slows down, none of your body parts -- from your eyes to your toes -- get enough oxygen. This brings about the early symptoms of PV, including dizziness, itchiness, and headaches. Thicker blood is also more likely to form a clot -- a clump of blood that stops up a vein or artery. Blood clots can lead to life-threatening problems such as a heart attack or stroke. PV is a slow-growing cancer. You can go years without seeing symptoms. Your doctor can check for PV with a basic blood test, but most people with PV find out because they had the test for some other reason. There's no cure for PV, but there are treatments. Most people with PV live a normal life when they get the care they need. Doctors don't know what causes polycythemia vera. It's not linked to anything you do, the way smoking makes you more likely to get lung cancer. Anyone can get PV, but it's usually seen in people over 60. Men are a little more likely than women to get it. While the cause isn't clear, most people with PV have a problem in a gene called JAK2. Your bone marrow -- the spongy center part of your bone -- creates your blood cells. Normally, it makes just the right amount. But if your JAK2 gene doesn't work right, your bone marrow makes too many red blood cells. Even though the problem is in a gene, you don't get PV from your parents. The gene changes at some point after you're born, but doctors don't know why. Because PV grows slowly, you might have it for years without knowing it. When you do see symptoms, they may not seem all that unusual. In fact, they're the same as with many other illnesses: Dizziness Headache Itchiness, often after a warm bath or shower More sweating than normal, sometimes at night Shortness of breath or trouble breathing when you lie down Tiredness Weakness Brief vision problems, like seeing flashes You may also have: Bloating or a feeling of fullness on the upper left side of your belly Nosebleeds, bleeding gums, or more menstrual bleeding than normal Numbness, tingling, or burning in your hands and feet Problems with your vision, like seeing double or things seeming blurry Reddened face Swelling and pain in one joint, usually your big toe PV can lead to other issues. But your doctor will work to avoid those problems. Blood clots are the most serious concern because they can cause a stroke, heart attack, or other life-threatening problems, such as a DVT (a blood clot in your legs) or a pulmonary embolism (a blood clot that travels to your lungs). Clots can also make your spleen and liver larger than normal, giving you sharp pains in your belly. It's rare, but some people with PV get leukemia or another bone marrow illness called myelofibrosis. No, but many people with PV live a normal life span. With the right care, you can limit your symptoms and, in some cases, make them go away completely. The best treatment for you depends on your age, history, and how far along the PV is. Your doctor can help you make the best choices for your health now and in the years to come with follow-up care to make sure you don't have complications.