Blood clots in the leg can break off and travel to the lungs, which can result in a dangerous pulmonary embolism, according to Mayo Clinic. They can also result in a condition known as postphlebitic syndrome, which can appear even after the blood clot is removed.
A pulmonary embolism is a medical emergency, explains Mayo Clinic. The person suddenly has trouble catching his breath and has a pain in the chest that worsens when he coughs or takes a deep breath. He may feel lightheaded or pass out; have a rapid pulse; or cough up blood. In postphlebitic syndrome, which is also called postthrombotic syndrome, the person's legs swell and hurt. The skin discolors and develops sores. This is because of the damage the blood clot has left behind. A person may not experience these symptoms until years after he is treated for the blood clot.
People who are at higher risk from blood clots in the legs include those with an inherited blood-clotting disease, states Mayo Clinic. Others at risk include people who are immobilized for a long time, have had surgery, suffered an injury to the veins in their legs, are overweight, are pregnant or on birth control pills. Smokers, cancer patients, older people or people with a family history of blood clots in the legs are also at higher risk.