Factor V Leiden is a mutation of one of the clotting factors in the blood called factor V. This mutation can increase your chance of developing abnormal blood clots (thrombophilia), usually in your veins. Most people with factor V Leiden (FAK-tur five LIDE-n) never develop abnormal clots. But some do develop clots that lead to long-term health problems or become life-threatening. Both men and women can have factor V Leiden. Women may have an increased tendency to develop blood clots during pregnancy or when taking the hormone estrogen. If you have factor V Leiden and have developed blood clots, anticoagulant medications can lessen your risk of developing additional blood clots and help you avoid potentially serious complications.