Leg swelling happens when excess fluid builds up in the tissues of your lower extremities. The causes range from something as simple as sitting too long to serious conditions involving the heart, kidneys, or blood vessels. Whether one leg is swollen or both offers an important clue: one-sided swelling usually points to a local problem like a blood clot or vein issue, while swelling in both legs typically signals something systemic.
How Fluid Builds Up in Your Legs
Your body constantly moves fluid between your bloodstream and the surrounding tissues. This exchange stays balanced through a combination of blood pressure inside your capillaries, protein levels in your blood (which pull fluid back in), and your lymphatic system, which acts as a drainage network for excess fluid. Leg swelling develops when any part of this system tips out of balance.
Four things can go wrong. First, pressure inside the blood vessels can rise too high, pushing more fluid out into the tissues. This happens with heart failure or when blood pools in the veins. Second, protein levels in the blood can drop too low, which weakens the pull that keeps fluid inside the vessels. Third, the walls of the blood vessels can become leaky from injury or inflammation, letting fluid seep through. And fourth, the lymphatic drainage system can become blocked or damaged, leaving fluid with nowhere to go. Most cases of leg swelling involve one or more of these mechanisms.
One Leg vs. Both Legs
This distinction matters because it narrows down the likely cause. Unilateral swelling, meaning just one leg, favors problems with the veins or lymphatic system on that side. In people over 50, venous disease is the single most common cause of one-sided leg swelling. Other causes include deep vein thrombosis (a blood clot), a ruptured cyst behind the knee, muscle tears, fractures, sprains, and infections.
Bilateral swelling, both legs at once, usually points to a systemic condition affecting the whole body. Heart failure, kidney disease, liver disease, medication side effects, and obesity are the most frequent culprits. Pregnancy and hormonal changes before menstruation can also cause fluid retention in both legs. That said, the cause is often multifactorial, meaning more than one thing contributes.
Heart Failure
When the heart can’t pump blood efficiently, pressure builds up in the veins, and fluid gets pushed out into the tissues. But the swelling isn’t just a plumbing problem. Reduced blood flow triggers a cascade of hormonal responses designed to hold onto sodium and water, which only makes things worse. The kidneys, sensing lower blood flow, activate systems that cause the body to retain even more fluid. This creates a cycle: the weaker the heart pumps, the more fluid the body holds, and the more swollen the legs become.
Heart failure swelling tends to worsen over the course of the day and improves somewhat overnight when you’re lying flat. It’s typically accompanied by other symptoms like shortness of breath, fatigue, or difficulty breathing when lying down.
Venous Insufficiency and Blood Clots
Your leg veins have one-way valves that push blood upward against gravity. When these valves weaken or fail, blood pools in the lower legs, raising pressure in the veins and forcing fluid into surrounding tissue. This condition, chronic venous insufficiency, is the most common vascular cause of leg swelling. It often starts in one leg but frequently progresses to both over time.
A deep vein thrombosis is a blood clot that forms in one of the deep veins of the leg. It causes sudden swelling, usually in just one leg, along with warmth, redness, or pain. DVT requires urgent treatment because pieces of the clot can break off and travel to the lungs. Acute swelling that develops within 72 hours is more characteristic of DVT, infection, or trauma, while swelling that builds gradually over weeks or months points toward chronic venous insufficiency or lymphatic problems.
People with advanced venous disease often develop secondary lymphatic damage as well, meaning the two conditions overlap. Venous disease, not cancer, is now recognized as the most common cause of lymphedema in Western countries.
Kidney and Liver Disease
The kidneys regulate how much sodium and water your body retains. When they’re damaged, they may allow too much protein to leak into the urine, lowering the protein concentration in the blood. This weakens the force that pulls fluid back into the bloodstream, so it accumulates in the tissues instead. Kidney disease also impairs the body’s ability to excrete excess sodium, compounding the problem.
Liver disease, particularly cirrhosis, causes swelling through a similar mechanism. A damaged liver can’t produce enough albumin, the main protein responsible for keeping fluid inside blood vessels. The resulting drop in blood protein levels leads to fluid leaking out into the legs and abdomen. Malnutrition and conditions that impair nutrient absorption can produce the same effect.
Medications That Cause Swelling
Several common medications list leg swelling as a side effect, but calcium channel blockers, a class of blood pressure drugs, are the most frequent offenders. These medications relax blood vessel walls, which can increase the pressure inside capillaries and push fluid into surrounding tissues. A meta-analysis of randomized trials found that about 10.7% of people taking calcium channel blockers developed peripheral edema, compared to 3.2% on placebo. At higher doses, the rate climbed to 16.1%, and after six months of use, nearly one in four patients experienced some degree of swelling.
Not all calcium channel blockers carry equal risk. The type known as dihydropyridines caused swelling in 12.3% of patients, while other subtypes caused it in only 3.1%. Over the long term, more than 5% of patients stopped taking these drugs specifically because of swelling. Other medications that can cause leg edema include certain diabetes drugs, anti-inflammatory painkillers, steroids, and some hormone therapies.
Lymphedema
The lymphatic system is your body’s secondary drainage network, collecting excess fluid from tissues and returning it to the bloodstream. When lymphatic vessels are blocked, damaged, or missing, fluid accumulates and produces a distinctive type of swelling. Lymphedema can result from surgery (especially lymph node removal during cancer treatment), radiation, infection, tumors, or congenital abnormalities.
Lymphedema looks and feels different from other types of swelling. The skin often becomes firm, thick, or “brawny” rather than soft and squishy. A useful test: if you try to pinch the skin at the base of the second toe and can’t lift a fold, that’s a positive Stemmer’s sign, which strongly suggests lymphedema. Unlike the pitting edema seen with heart or kidney problems, where pressing a finger into the skin leaves a temporary dent, lymphedema tends to be non-pitting, especially as it progresses.
Pitting vs. Non-Pitting Edema
When you press your finger into a swollen area for several seconds, the response tells you something about the cause. Pitting edema leaves an indentation that slowly fills back in. It’s graded on a four-point scale based on depth and recovery time:
- Grade 1: A 2 mm dent that rebounds immediately
- Grade 2: A 3 to 4 mm dent that rebounds in under 15 seconds
- Grade 3: A 5 to 6 mm dent that takes 15 to 60 seconds to rebound
- Grade 4: An 8 mm dent that takes two to three minutes to rebound
Pitting edema is typical of heart failure, kidney disease, liver disease, venous insufficiency, and medication-related swelling. Non-pitting edema, where the skin doesn’t indent easily, is more characteristic of lymphedema or thyroid disease.
Other Contributing Factors
Obesity increases pressure on the veins in the pelvis and legs, making it harder for blood to return to the heart. It also compresses lymphatic channels and can contribute to obstructive sleep apnea, which itself worsens fluid retention. Prolonged sitting or standing allows gravity to pull fluid into the lower legs, a phenomenon called occupational edema that affects many people with desk jobs or jobs that keep them on their feet.
Lipedema, a condition often confused with obesity or lymphedema, causes symmetric fat deposits in the legs that can be painful and don’t respond to diet or exercise. It almost exclusively affects women and is frequently underdiagnosed. Pregnancy, spinal cord injuries, and thyroid disorders round out the list of common contributors.
Reducing Everyday Swelling
For mild swelling related to prolonged sitting or standing, compression stockings are the most studied solution. Light compression in the 10 to 15 mmHg range is effective at preventing occupational edema. Stockings in the 15 to 20 mmHg range produce significant reduction within a couple of days, and 20 to 30 mmHg stockings offer even more benefit, particularly for people who sit most of the day. Higher pressure is not always better for prevention; research suggests that light pressure works well and higher levels may not add additional benefit for otherwise healthy legs.
Elevating your legs above heart level, reducing sodium intake, staying active, and taking breaks from sitting or standing all help minimize fluid pooling. For swelling caused by an underlying condition, the swelling typically improves when the root cause is treated.
When Leg Swelling Is an Emergency
Most leg swelling develops gradually and isn’t immediately dangerous, but certain combinations of symptoms require emergency attention. Leg swelling paired with chest pain, difficulty breathing, shortness of breath when lying flat, fainting, dizziness, or coughing up blood can indicate a blood clot in the lungs or a serious cardiac event. Sudden, severe swelling in one leg with warmth and pain also warrants urgent evaluation for DVT, since early treatment significantly reduces the risk of complications.