Leg swelling happens when fluid builds up in the tissues of your lower legs, ankles, or feet. The causes range from something as simple as sitting too long to serious conditions involving your heart, kidneys, or veins. Whether one leg is swollen or both, and whether the swelling came on suddenly or gradually, tells a lot about what’s behind it.
How Fluid Ends Up in Your Legs
Your body constantly moves fluid between your bloodstream and the tissues around it. Two main forces keep this in balance: pressure inside your blood vessels pushes fluid out, while proteins in your blood (especially one called albumin) pull fluid back in. Your lymphatic system also acts as a drainage network, collecting excess fluid from tissues and returning it to your bloodstream.
Swelling happens when any part of this system breaks down. That could mean too much pressure pushing fluid out, not enough protein pulling it back in, leaky blood vessel walls, or a lymphatic system that can’t keep up with drainage. Nearly every cause of leg swelling traces back to one of these four mechanisms.
Venous Insufficiency
This is one of the most common reasons for chronic leg swelling, particularly in people over 50. Your leg veins contain one-way valves that push blood upward toward your heart against gravity. When those valves become damaged, blood pools in the lower legs, increasing pressure inside the veins and forcing fluid into the surrounding tissue.
The swelling from venous insufficiency tends to be worst at the end of the day or after long periods of standing. Over time, the condition progresses through stages. Early on, you might notice only tired, achy legs with visible spider veins. As it advances, persistent swelling develops in the lower legs and ankles, and the skin can become discolored, thickened, or prone to slow-healing ulcers.
Heart Failure
When the right side of the heart can’t pump blood efficiently, it backs up in the veins. That backup increases pressure in the blood vessels of the legs and abdomen, pushing fluid into the tissues. The result is swelling in both legs, feet, and sometimes the belly. This type of swelling often worsens over the course of the day and improves overnight when you’re lying flat, though in more advanced cases it may be present all the time.
Heart-related swelling is almost always in both legs equally. If you notice it alongside shortness of breath, fatigue, or difficulty lying flat to sleep, those are signs the swelling could be cardiac in origin.
Kidney and Liver Disease
Your kidneys regulate how much fluid and salt your body retains. When they’re not working well, sodium and water build up, expanding your blood volume and increasing pressure in your vessels. Kidney disease can also cause your body to lose albumin through urine. Since albumin is the main protein that holds fluid inside blood vessels, losing it allows fluid to leak into your tissues.
Liver disease, particularly cirrhosis (severe scarring), contributes to swelling through multiple pathways at once: the damaged liver produces less albumin, fluid backs up in veins that pass through the liver, and kidney function often deteriorates as a secondary effect. People with cirrhosis commonly develop swelling in both the abdomen and legs.
Blood Clots
A deep vein thrombosis (DVT) is a blood clot that forms in a deep vein, usually in one leg. It blocks blood flow and causes a sudden increase in pressure below the clot. The key difference from most other causes is that DVT typically affects only one leg. Warning signs include:
- Sudden swelling in one leg (not both)
- Pain or tenderness that may worsen when walking or standing
- Warmth in the swollen area
- Skin that looks red or discolored
A DVT requires urgent medical attention because the clot can break loose and travel to the lungs, causing a pulmonary embolism. Some people with DVT have mild symptoms or none at all, which makes sudden one-sided leg swelling something to take seriously even if it doesn’t hurt much.
Medications That Cause Swelling
Several common prescription drugs list leg swelling as a side effect, and it’s a frequent enough problem that many patients stop taking these medications because of it.
Blood pressure medications in the calcium channel blocker family are among the most common culprits. They work by relaxing blood vessels, but they dilate the small arteries feeding into capillaries more than the veins draining them. This mismatch increases pressure inside the capillaries and pushes fluid into the surrounding tissue.
Anti-inflammatory painkillers (both over-the-counter types like ibuprofen and prescription versions) cause the kidneys to retain more sodium and water, which expands blood volume and raises pressure in the vessels. Steroids do something similar through their effect on kidney salt handling. Diabetes medications in the thiazolidinedione class, certain nerve pain drugs, and insulin can also cause fluid retention. If your leg swelling started or worsened after beginning a new medication, that connection is worth exploring with your prescriber.
Lymphedema
When the lymphatic drainage system itself is damaged or blocked, protein-rich fluid accumulates in the tissues. This produces a distinct type of swelling that, over time, causes the skin to harden and thicken, a change called fibrosis. The affected limb can feel heavy and tight, and recurring skin infections are common.
The most frequent cause of lymphedema in developed countries is cancer treatment. Surgery that removes lymph nodes or radiation that scars lymph vessels can permanently impair drainage, sometimes not appearing until months or years after treatment. In tropical regions, parasitic infections that clog the lymph nodes are the leading cause. Less commonly, people are born with an underdeveloped lymphatic system that causes swelling later in life.
Lymphedema swelling doesn’t respond to simple elevation the way venous swelling does, and pressing on the skin may not leave much of an indent, especially as the condition progresses. This makes it feel and behave differently from the fluid retention caused by heart or vein problems.
Pitting vs. Non-Pitting Swelling
One quick test you can do at home: press your thumb firmly into the swollen area for about 10 seconds and then release. If a dent remains for several seconds before filling back in, that’s called pitting edema. It usually points to fluid overload from heart, kidney, liver, or vein problems.
If the skin bounces right back with little or no dent, that’s non-pitting edema. This pattern is more typical of lymphedema or thyroid conditions, where the fluid is thicker and bound up with proteins. The distinction isn’t perfect, but it gives you and your doctor useful information about where to look for the cause.
Pregnancy-Related Swelling
Some swelling in the legs and ankles is normal during pregnancy, especially in the third trimester, as the growing uterus puts pressure on the veins returning blood from the legs. But sudden swelling, particularly in the face and hands along with the legs, can signal preeclampsia. The defining features of preeclampsia are high blood pressure and signs of organ stress, especially in the kidneys. Sudden weight gain or a rapid increase in swelling during pregnancy warrants a prompt blood pressure check.
How the Cause Is Identified
Doctors start by looking at whether the swelling is in one leg or both, how quickly it developed, and what other symptoms are present. One-sided swelling raises concern for a blood clot or local vein problem. Swelling in both legs points more toward a systemic issue like heart failure, kidney disease, or medication effects.
A duplex ultrasound of the leg veins is the most common first imaging test. It can reveal blood clots, valve damage, and blood flow patterns without any radiation. Blood tests help check for organ-related causes: kidney function markers, liver enzymes, albumin levels, and thyroid hormones can all point toward a specific explanation. A blood test called D-dimer helps rule out acute blood clots when the risk seems low. If heart failure is suspected, a blood marker that rises when the heart is under strain can help confirm or rule it out.
Managing Everyday Swelling
For swelling related to prolonged sitting or standing, venous insufficiency, or mild fluid retention, compression stockings are the frontline approach. Research shows that stockings providing 10 to 15 mmHg of pressure can effectively prevent swelling in people who sit or stand for long hours at work. Stockings in the 15 to 20 mmHg range are widely available without a prescription and work well for most mild to moderate swelling. Higher-pressure garments (20 to 30 mmHg) may be recommended for more significant venous problems but aren’t necessarily more effective for everyday use.
Elevating your legs above the level of your heart for 20 to 30 minutes a few times a day helps gravity move fluid back toward your core. Regular movement matters too: the calf muscles act as a pump that squeezes blood upward through the veins, so even short walks or calf raises throughout the day can make a meaningful difference. Reducing salt intake helps limit how much fluid your body retains in the first place.
These strategies work well for benign causes, but swelling that’s sudden, one-sided, painful, or accompanied by shortness of breath, chest pain, or significant skin changes points to something that needs medical evaluation rather than home management.