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The extra red blood cells make your blood thicker than normal. As a result, blood clots can form more easily. These clots can block blood flow through your arteries and veins, which can cause a heart attack or stroke. Thicker blood also doesn't flow as quickly to your body as normal blood.
Cancer can also cause blood thickening. Not all blood clots are caused by thick blood. Blood clots leading to heart attacks and strokes can be formed when thin blood comes into contact with plaque, which triggers a blood clot. Some blood clots result from sluggish blood flow or poor circulation.