A blood clot (thrombus) normally forms to stop the bleeding when an artery or vein is damaged, such as when you experience a cut. Clots form as a result of clotting factors in your blood. There are also substances in your blood known as anti-clotting factors to control excessive formation of blood clots.

Factor V Leiden is a mutation of one of the clotting factors in the blood called factor V (Factor 5). This mutation can increase your chance of developing abnormal blood clots (thrombophilia), usually in your veins.

People with factor V Leiden have a genetic mutation that causes the factor V protein to respond more slowly to being deactivated by the anti-clotting factors. This makes Factor V to stay longer in the blood and increases the chance of clotting.

If you have factor V Leiden, you either inherited one copy of the defective gene (heterozygous), which slightly increases your risk of developing blood clots, or more rarely you inherited two copies, one from each parent (homozygous), which significantly increases your risk of developing blood clots.

Both men and women can have factor V Leiden, but women may have an increased tendency to develop blood clots during pregnancy or when taking the hormone estrogen.

Persons with thrombophilia are at increased risk of developing clots that obstruct blood flow locally or that detach and embolize. Environmental factors that cause vascular injury (e.g., surgery), stasis (e.g., prolonged immobility), or increased coagulability (e.g., hormone use) interact with genetic susceptibility to increase the risk for thrombosis.