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. Most people who have factor V Leiden never develop signs or symptoms. The first indication that you have the disorder may be the development of a blood clot (thrombosis). Some clots do no damage and disappear on their own. Others can be life-threatening. Symptoms of a blood clot depend on where it forms and whether and where it travels. A clot in a deep vein this is known as deep vein thrombosis (DVT).