Causes of swollen ankles or feet include congestive heart failure, pregnancy, injury to the ankle or foot, kidney disease and sitting or being on one's feet for a long time, according to WebMD. If the person's ankles swell in the evening, it could also be a sign the body is retaining salt.
Certain medications such as oral contraceptives can also cause swollen feet and ankles, says WebMD. Other hormone-based medications can cause swelling, as can some antidepressants, calcium channel blockers, NSAIDs and diabetes medications. Congestive heart failure causes the ankles and feet to swell because the heart is not pumping as much blood to the kidneys. This causes fluid retention, which results in the ankles and other areas of the body swelling up. Kidney failure can also cause swollen feet and ankles, again because of fluid retention.
Though some swelling of the ankles in pregnancy is normal, if the ankles start to swell quickly, it may be a sign of eclampsia, explains WebMD. This happens after the 20th week of pregnancy and is a sign that blood pressure is dangerously high and protein is being excreted in the urine. A sprained ankle is a common reason for the ankle to swell. A sprained ankle usually responds well to the RICE formula of rest, ice, compression and elevation.