A slow-growing blood cancer in which bone marrow makes too many red blood cells.

Polycythemia vera is a rare, slow-growing cancer that mainly affects people over 60. Bone marrow makes too many red blood cells, and sometimes white blood cells and platelets too. The extra cells can cause blood clots. It is often found by chance in a blood test done for another reason.

Many people with polycythemia vera don't have symptoms. Some people have headaches, dizziness, tiredness and blurred vision. Itchiness can happen, especially after a warm bath or shower. There might be numbness or tingling in the hands, arms, feet or legs and swelling of a joint, often the big toe.

The most common treatment is to have blood withdrawn often. Medicines can lower the number of red blood cells. Medicines and treatment with a special light can ease itching.