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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.
Anything that damages the inner lining of a vein may cause DVT -- surgery, an injury, or your immune system. If your blood is thick or flows slowly, it's more likely to form a clot, especially in a vein that's already damaged.