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 on a flight to serious conditions like heart failure or a blood clot. Understanding what’s behind the swelling, whether it affects one leg or both, and what other symptoms accompany it can help you figure out how urgent the situation is.
How Fluid Builds Up in Your Legs
Your body constantly moves fluid between your blood vessels and the surrounding tissues. Swelling occurs when that balance tips in one of several ways: pressure inside your blood vessels rises too high, pushing fluid out; proteins in your blood drop too low, reducing the pull that keeps fluid inside vessels; the walls of your blood vessels become too leaky; or your lymphatic system, which normally drains excess fluid back into circulation, gets blocked or damaged. Most causes of leg swelling trace back to one or more of these mechanisms.
The distinction between one-leg and two-leg swelling is one of the most useful clues. Swelling in both legs typically points to a systemic problem, something affecting your whole body like heart, kidney, or liver disease. Swelling in just one leg raises concern for a local issue: a blood clot, an injury, or a problem with the veins or lymphatic vessels on that side.
Chronic Venous Insufficiency
Chronic venous insufficiency (CVI) is one of the most common causes of persistent leg swelling. It develops when the valves inside your leg veins stop working properly, allowing blood to flow backward and pool in the lower legs instead of returning efficiently to the heart. Varicose veins, which affect between 5% and 30% of adults, are an early visible sign. Studies using ultrasound have found CVI in roughly 9% of men and 7% of women.
The swelling from CVI tends to worsen over the course of the day, especially if you spend long periods standing or sitting. It often improves overnight when your legs are elevated. Over time, the sustained pressure in your veins can cause skin changes: brownish discoloration from iron deposits leaking out of blood cells, a type of eczema called stasis dermatitis, and hardening of the tissue around your ankles. If left untreated, about 20% of people with CVI eventually develop venous ulcers, open wounds that are slow to heal.
Heart Failure
When your heart can’t pump blood effectively, fluid backs up in your circulatory system and collects in your lungs, legs, feet, and abdomen. Heart failure is a major cause of bilateral leg swelling, and the swelling often comes with other telltale symptoms: shortness of breath (particularly when lying flat or during the night), fatigue with activity, a dry persistent cough, sudden unexplained weight gain, and a bloated or hard stomach.
The leg swelling from heart failure is typically worse at the end of the day and may leave a visible dent when you press on the skin, known as pitting edema. If you notice new or worsening swelling in both legs along with shortness of breath at rest or rapid weight gain over a few days, that combination warrants prompt medical attention.
Blood Clots
A deep vein thrombosis (DVT) is a blood clot that forms in one of the deep veins of your leg. It’s one of the more urgent causes of leg swelling because pieces of the clot can break off, travel to the lungs, and cause a pulmonary embolism, which can be life-threatening.
DVT typically causes swelling in just one leg, along with pain or cramping (often starting in the calf), warmth in the affected area, and skin that looks red or purple. Risk factors include recent surgery, prolonged immobility (long flights, bed rest), cancer, pregnancy, and use of hormonal birth control. The swelling usually comes on over hours to days, not instantly, and the leg may feel heavy or tight.
Kidney and Liver Disease
Your kidneys and liver both play critical roles in keeping fluid where it belongs. When either organ fails, swelling can show up in the legs, feet, and sometimes the face and hands.
Kidney disease can cause a condition called nephrotic syndrome, where damaged filters in the kidneys leak large amounts of protein into the urine. When blood albumin (the main protein that holds fluid inside your blood vessels) drops below about 2.5 grams per deciliter, there isn’t enough pulling force to keep fluid in the bloodstream. It seeps into surrounding tissues, causing widespread puffiness. This swelling often appears around the eyes in the morning and shifts to the legs and ankles later in the day.
Liver disease, particularly cirrhosis, causes a similar drop in albumin because the liver produces it. Liver-related swelling commonly involves the abdomen (a condition called ascites) alongside the legs. It’s often accompanied by jaundice, easy bruising, and fatigue.
Medications That Cause Swelling
Several commonly prescribed drug classes can cause leg swelling as a side effect. Calcium channel blockers, a widely used type of blood pressure medication, are among the most frequent culprits. These drugs relax blood vessel walls, which can increase fluid leakage into surrounding tissues, particularly in the ankles and feet. The swelling tends to develop gradually after starting the medication and may worsen at higher doses.
Other medications linked to leg swelling include:
- Anti-inflammatory painkillers (NSAIDs) like ibuprofen and naproxen, which cause your kidneys to retain sodium and water
- Corticosteroids such as prednisone, especially with prolonged use
- Diabetes medications in the thiazolidinedione class
- Hormonal therapies including estrogen-containing birth control and hormone replacement
- Some antidepressants and gabapentin-type nerve pain medications
If you notice new leg swelling after starting a medication, it’s worth checking whether the timing lines up. Switching to a different drug in the same class can sometimes resolve the problem.
Lymphedema
Lymphedema occurs when your lymphatic system, the network of vessels that drains excess fluid from tissues, is damaged or blocked. This causes a distinctive type of swelling that differs from other forms in important ways: it doesn’t pit easily when pressed, the skin gradually thickens and becomes firm, and it often affects one limb more than the other.
Common causes include surgery that removes lymph nodes (particularly after cancer treatment), radiation therapy, severe infections, and obesity. In some cases lymphedema is inherited. One simple physical test can help identify it: if you try to pinch and lift the skin on top of your second toe and the skin is too thick and stiff to grab, that’s a positive Stemmer sign, which strongly suggests lymphedema. The thickening comes from chronic inflammation and scarring in the tissues caused by protein-rich fluid that the lymphatic system can’t clear.
Pregnancy-Related Swelling
Some degree of leg and ankle swelling is normal during pregnancy, especially in the third trimester, as your growing uterus compresses the veins returning blood from your legs. Hormonal changes also cause your body to retain more fluid. This type of swelling is usually mild, affects both legs equally, and worsens in hot weather or after standing for long periods.
Swelling becomes concerning when it’s sudden, severe, or appears in the hands and face. These are potential signs of preeclampsia, a serious complication diagnosed after 20 weeks of pregnancy when blood pressure reaches 140/90 mmHg or higher along with protein in the urine. Preeclampsia can escalate quickly and requires immediate medical management. Headaches, vision changes, and upper abdominal pain alongside sudden swelling are red flags during pregnancy.
How Pitting Edema Is Graded
When a doctor presses a finger into swollen tissue and it leaves an indentation, that’s pitting edema. The depth of the dent and how long it takes to bounce back tell your doctor how severe the fluid buildup is:
- Grade 1: A 2 mm dent that rebounds immediately. Mild swelling.
- Grade 2: A 3 to 4 mm dent that fills back in within 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 persists for two to three minutes. Severe swelling.
Not all swelling pits. Lymphedema, for instance, typically produces non-pitting edema once it progresses beyond the early stages. The type of swelling and how it behaves under pressure helps narrow down the cause.
When Leg Swelling Is an Emergency
Most causes of leg swelling develop gradually and aren’t immediately dangerous. But certain combinations of symptoms signal a medical emergency. Call 911 if leg swelling occurs alongside chest pain, difficulty breathing, shortness of breath when lying down, fainting, dizziness, or coughing up blood, as these may indicate a blood clot in the lungs or a serious cardiac event.
You should also seek same-day medical care if your swelling comes on suddenly without an obvious cause, follows a physical injury like a fall or car accident, or appears in only one leg with pain, coolness, or pale skin. One-sided swelling with pain and warmth raises concern for DVT and needs evaluation before a clot can travel to the lungs.