Potential causes for swelling in one leg include infection or internal bleeding as a result of an injury, according to Drugs.com. Causes for swelling in only one leg are typically not the same for swelling in both legs.
A common cause of swelling in the legs is a blood clot that forms in the deep veins of the body, or deep vein thrombosis, according to the Mayo Clinic. The condition is generally caused by lack of regular movement, such as being confined to bed after an injury. Many patients don't experience any symptoms. In some cases, the blood clot breaks loose and travels through the body up to the lungs causing a pulmonary embolism, which is a life-threatening condition that requires emergency medical attention. Symptoms of a pulmonary embolism include chest pain, fainting, sudden shortness of breath, rapid pulse and coughing up blood.
Other causes of swelling in one leg include venous insufficiency, varicose veins, insect bites and external pressure placed on the leg from garters or stockings, according to University of Minnesota Medical Center. Effective treatment depends on the cause of the swelling, but it may include wearing loose clothing, keeping feet and legs elevated when lying down or sitting, and taking frequent short walks to boost blood circulation. A doctor may prescribe medication or recommend support stockings to help reduce swelling.