A swollen leg can result from dozens of different causes, ranging from a minor muscle strain to a serious blood clot or heart problem. The single most important clue is whether one leg is swollen or both. One-sided swelling typically points to a local problem in that leg, like a clot, injury, or infection. Swelling in both legs usually signals something systemic, like fluid retention from heart, kidney, or liver issues.
One Swollen Leg vs. Both
When only one leg swells up suddenly, the most common cause is actually a muscle strain, tear, or twisting injury, accounting for roughly 40% of cases. A deep vein blood clot (DVT) is the most urgent possibility and needs to be ruled out first, but it’s not the most frequent explanation. Other causes of one-sided swelling include infection, a fluid-filled cyst behind the knee (Baker’s cyst), vein problems, and blocked lymph drainage.
When both legs swell, the cause is more likely to be a whole-body issue. Heart failure is one of the most common, but kidney disease, liver disease, and certain medications can all do it. Hormonal shifts before a menstrual period, sleep apnea, and even prolonged sitting or standing can contribute to bilateral swelling. In pregnancy, mild swelling in both legs is normal, but sudden or severe swelling can signal a complication called preeclampsia.
Blood Clots: The Risk You Shouldn’t Ignore
A DVT forms when blood clots inside a deep vein, usually in the calf or thigh. The leg often feels warm, tender along the inner thigh or calf, and may look reddish or discolored. The swelling typically develops over hours to a couple of days and affects just one leg. If the calf of the swollen leg is more than 3 centimeters larger than the other side, that raises suspicion considerably.
Doctors assess DVT risk by looking at a combination of factors: recent surgery or immobilization, active cancer treatment, a history of previous clots, and whether the entire leg is swollen. People with several of these factors have a probability as high as 53% of actually having a clot. Those with none of them have about a 5% chance.
The danger with a DVT is that part of the clot can break off and travel to the lungs, causing a pulmonary embolism. If you have leg swelling along with sudden shortness of breath, chest pain that worsens when you breathe deeply, a rapid or irregular heartbeat, coughing up blood, or feeling faint, that combination requires emergency care immediately.
Venous Insufficiency and Skin Changes
Chronic venous insufficiency is one of the most common reasons for long-term leg swelling. It happens when the one-way valves inside your leg veins weaken, allowing blood to pool rather than flow back up toward the heart. The swelling tends to worsen through the day and improve overnight. You might first notice spider veins, then larger varicose veins, and eventually persistent swelling.
Left untreated, the pressure inside the veins builds until the smallest blood vessels burst. This causes the skin to take on a reddish-brown discoloration, particularly around the ankles. Over time the skin can become leathery, flaky, and itchy. In advanced stages, scar tissue traps fluid in the tissues, making the calf feel large and hard. The skin becomes fragile and prone to open sores (ulcers) that are slow to heal. The condition is staged from 0 to 6: stage 0 involves only achiness and tiredness in the legs, while stage 6 means an active ulcer is present.
Heart, Kidney, and Liver Problems
When the heart can’t pump efficiently, blood flow slows down and backs up in the veins returning to the heart. Fluid then leaks out of blood vessels and collects in the tissues, especially in the legs and ankles because gravity pulls it there. This is one of the hallmark signs of heart failure, and the swelling is usually in both legs. It often comes with other symptoms like shortness of breath, fatigue, and waking up at night to urinate.
Kidney disease causes swelling through a different mechanism. The kidneys lose their ability to filter excess fluid and salt, so the body retains water. Liver disease, particularly cirrhosis, reduces the production of proteins that help keep fluid inside blood vessels. With fewer of those proteins circulating, fluid seeps into surrounding tissues. Both conditions produce swelling that tends to affect both legs symmetrically and may also show up in the hands, face, or abdomen.
Medications That Cause Swelling
Several common medications list leg or ankle swelling as a side effect. Calcium channel blockers, a widely prescribed class of blood pressure drugs, are among the most frequent culprits. The swelling occurs because these drugs relax blood vessel walls, which allows more fluid to leak into surrounding tissue. The incidence ranges from 1% to 15% of patients, and it’s more common with certain types like amlodipine, nifedipine, and felodipine. Hormone therapies (including birth control and hormone replacement), certain diabetes medications, and some anti-inflammatory drugs can also cause fluid retention in the legs.
If your swelling started shortly after beginning a new medication, that timing is a strong clue. Don’t stop the medication on your own, but the connection is worth raising with your prescriber.
Lymphedema: When Drainage Is Blocked
Your lymphatic system acts like a secondary drainage network, clearing excess fluid and proteins from tissues. When that system is damaged or blocked, protein-rich fluid accumulates and causes a distinctive type of swelling called lymphedema. It often affects just one leg and tends to involve the top of the foot, giving a puffy, squared-off appearance.
In its early stages, lymphedema may look and feel like regular swelling, and pressing on the skin may still leave a temporary dent. Over time, though, the stagnant fluid triggers the tissue to thicken and become fibrous. The swelling becomes firm rather than squishy and no longer leaves an indent when pressed. The skin may thicken, develop a rough or warty texture, and feel hard. One quick clue: if you can’t pinch a fold of skin at the base of the second toe on the swollen foot, that test is about 92% sensitive for lymphedema.
A key difference from other types of swelling is that lymphedema typically does not improve much with leg elevation, while swelling from vein problems or heart issues often does.
The Pitting Test
You can do a simple check at home. Press your thumb firmly into the swollen area for about 10 seconds, then release. If a visible dent remains and slowly fills back in, that’s pitting edema. It generally indicates fluid overload from heart failure, vein insufficiency, kidney or liver disease, or medication side effects.
If the skin springs back immediately with no dent, or if the tissue feels firm and rubbery, that’s non-pitting edema. This pattern is more typical of lymphedema or thyroid disease. Both types warrant evaluation, but they point your doctor in different diagnostic directions.
Pregnancy-Related Swelling
Mild, even swelling in both legs during the second and third trimesters is extremely common and usually harmless. The growing uterus compresses veins returning blood from the legs, and hormonal changes increase overall fluid volume.
The red flag is sudden or severe swelling, particularly after 20 weeks, combined with high blood pressure (140/90 or above). That pattern raises concern for preeclampsia, a potentially dangerous pregnancy complication. Preeclampsia is diagnosed when high blood pressure appears alongside signs of organ stress, such as protein spilling into the urine. Headaches, vision changes, and upper abdominal pain alongside new swelling are reasons to contact your provider promptly.
What Helps Reduce Swelling at Home
For mild swelling without alarming symptoms, a few strategies can make a meaningful difference. Elevating your legs above the level of your heart for periods throughout the day helps gravity move fluid back toward your core. Reducing salt intake limits how much water your body retains. Moving regularly, even short walks or calf raises, activates the muscle pumps in your legs that push blood and fluid upward.
Compression stockings apply graduated pressure that supports your veins and prevents fluid from pooling. Low-pressure stockings (under 20 mmHg) are available over the counter and are a reasonable starting point for mild swelling. Stockings rated at 20 mmHg or higher require a prescription and are typically used for diagnosed venous insufficiency or lymphedema. The right compression level depends on the cause of your swelling, so getting a diagnosis first matters.
These measures help manage symptoms, but they don’t replace figuring out the underlying cause. Swelling that’s new, one-sided, painful, rapidly worsening, or accompanied by skin changes, shortness of breath, or chest pain needs medical evaluation rather than home management alone.