Leg swelling happens when fluid builds up in the tissues of your lower extremities, and the causes range from something as simple as sitting too long to serious conditions like heart failure or blood clots. About 19% of older U.S. adults have chronic leg swelling, making it one of the most common health complaints in that age group. Understanding whether your swelling is harmless or a warning sign depends on a few key details: whether it affects one leg or both, how quickly it appeared, and what other symptoms come with it.
How Fluid Ends Up in Your Legs
Your body constantly moves fluid between your bloodstream and the surrounding tissues. Two forces keep this in balance: pressure inside your blood vessels pushes fluid out, while proteins (especially one called albumin) pull fluid back in. Your lymphatic system acts as a cleanup crew, draining whatever excess fluid leaks through. When any part of this system breaks down, fluid pools in the lowest point gravity can take it: your legs and feet.
There are essentially two ways swelling starts. First, something changes in your blood vessels that lets too much fluid leak out, whether that’s higher pressure, leakier vessel walls, or not enough protein to pull fluid back. Second, your kidneys start holding onto extra salt and water as a compensation response, which adds even more fluid to the system. Most causes of leg swelling involve one or both of these mechanisms.
One Leg vs. Both Legs
This distinction matters more than almost any other detail. Swelling in both legs at the same time usually points to a systemic problem, something affecting your whole body like heart, kidney, or liver disease. Swelling in just one leg is more likely a local issue: a blood clot, an injury, an infection, or a problem with the veins or lymphatic vessels on that side.
Asymmetric swelling that develops over months or years typically signals chronic venous or lymphatic disease. Sudden swelling in one leg, especially with pain, warmth, or redness, raises concern for a deep vein thrombosis (DVT), which requires urgent evaluation.
Venous Insufficiency: The Most Common Culprit
Chronic venous insufficiency (CVI) is by far the leading cause of persistent leg swelling. An estimated 10% to 35% of U.S. adults have it, with rates as high as 40% in women. It happens when the one-way valves inside your leg veins stop working properly, allowing blood to flow backward and pool. Over time, the increased pressure forces fluid out of the veins and into surrounding tissues.
CVI progresses through recognizable stages. It may start with spider veins or small visible veins, then progress to varicose veins, then swelling, then skin changes like darkening or thickening near the ankles. In advanced cases, the skin can break down into open sores called venous ulcers, which affect 1% to 3% of the U.S. population. Risk factors include older age, obesity, prolonged standing or sitting, prior blood clots, and family history. The prevalence is notably higher in industrialized countries, likely due to more sedentary lifestyles.
Heart Failure and Kidney Disease
When your heart can’t pump blood effectively, pressure builds up in the veins that return blood from the legs. At the same time, reduced blood flow to the kidneys triggers a cascade of hormonal signals that tell your body to hold onto sodium and water. Your nervous system ramps up, your kidneys reabsorb more salt at multiple points along their filtering tubes, and hormones like aldosterone drive even more fluid retention. The result is a vicious cycle: the weaker the heart pumps, the more fluid the body retains, and the worse the swelling gets.
Kidney disease causes swelling through a different path. When the kidneys’ filtering units are damaged, protein leaks into the urine. Losing large amounts of protein drops albumin levels in the blood, which means less pulling force to keep fluid inside blood vessels. Fluid seeps into tissues throughout the body, but gravity concentrates it in the legs. Severe cases can cause swelling everywhere, including the face and abdomen.
Liver disease, particularly cirrhosis, creates a similar problem. A scarred liver produces less albumin and increases pressure in the veins draining the abdomen. This can cause both abdominal fluid buildup and leg swelling, sometimes severe enough to be confused with kidney disease.
Blood Clots: The Urgent Concern
A deep vein thrombosis forms when blood clots inside one of the large veins in your leg, partially or completely blocking blood flow. The classic signs include calf pain or tenderness, swelling with pitting (where pressing on the skin leaves an indent), increased warmth, and sometimes a bluish tint to the skin. The swelling stays below the knee if the clot is in a lower leg vein but can extend up to the groin if a larger vein is involved.
Risk factors that raise suspicion include active cancer, recent surgery requiring anesthesia, immobilization for more than three days, paralysis, and a calf that measures more than 3 cm larger than the other side. DVT matters because a clot can break loose and travel to the lungs, causing a pulmonary embolism. Any sudden, unexplained swelling in one leg deserves prompt medical attention.
Medications That Cause Swelling
Several common drug classes can cause leg swelling, and many people don’t realize their medication is the source. The most well-known offenders are calcium channel blockers, a type of blood pressure medication. These drugs relax the small arteries feeding your capillaries without equally relaxing the veins, which increases pressure inside the capillaries and pushes fluid out into the tissues. The swelling tends to affect both ankles and gets worse over the course of the day.
Other medications linked to leg swelling include:
- Anti-inflammatory drugs (NSAIDs) like ibuprofen and naproxen, which promote salt and water retention by affecting kidney blood flow
- Steroids, which cause the kidneys to hold onto salt through their hormone-like activity
- Diabetes medications in the thiazolidinedione class, which increase both blood vessel leakiness and kidney fluid retention
- Nerve pain medications like gabapentin and pregabalin, which relax small arteries and raise capillary pressure
- Some blood pressure medications including certain ACE inhibitors, which can increase vessel wall leakiness through buildup of a natural compound called bradykinin
- Antipsychotic medications, which dilate blood vessels through their effects on the nervous system
If you notice new swelling after starting a medication, it’s worth discussing with your prescriber. In many cases, switching to a different drug in the same class resolves the problem.
Pregnancy Swelling and Warning Signs
Mild swelling in the feet and ankles during pregnancy is normal, especially in the third trimester. The growing uterus compresses pelvic veins, increased blood volume raises pressure throughout the vascular system, and hormonal changes make blood vessels more relaxed. Most pregnancy-related swelling is harmless and resolves after delivery.
The concern is preeclampsia, a serious pregnancy complication defined by new high blood pressure (140/90 or above) developing after 20 weeks of pregnancy, along with signs of organ stress. While swelling alone doesn’t diagnose preeclampsia, sudden or severe swelling, particularly in the face and hands, combined with headaches that don’t respond to pain relief, vision changes, or upper abdominal pain, warrants immediate evaluation. Preeclampsia can develop even before obvious symptoms appear, which is why regular prenatal blood pressure checks are important.
Everyday Causes and Lifestyle Factors
Not all leg swelling signals disease. Prolonged sitting or standing causes fluid to pool in the legs simply because of gravity. This is common in people with desk jobs or those who stand for work, and it typically resolves overnight when the legs are elevated. Hot weather dilates blood vessels and can worsen the effect. High salt intake plays a direct role too: the American Heart Association recommends keeping sodium below 1,500 mg per day, though most Americans consume far more than that. Excess sodium triggers the kidneys to retain water, increasing overall fluid volume.
Obesity contributes to leg swelling through multiple pathways. Extra weight increases pressure on the pelvic and leg veins, impairs lymphatic drainage, and is associated with higher rates of venous insufficiency, heart failure, and kidney disease.
Managing Mild to Moderate Swelling
For swelling related to venous insufficiency, prolonged sitting, or other non-urgent causes, compression stockings are a first-line approach. Stockings rated at 15 to 20 mmHg of pressure significantly reduce fluid buildup in people who sit or stand for long periods. Higher-pressure stockings in the 20 to 30 mmHg range provide additional benefit, particularly for people who sit most of the day. Research has found that even light compression of 10 to 15 mmHg can prevent swelling from developing, though lower pressures than that tend to be ineffective.
Elevating your legs above heart level for 20 to 30 minutes several times a day helps gravity drain fluid back toward the core. Regular movement, even short walks or calf raises during the workday, activates the muscle pump in your lower legs that pushes blood back up through the veins. Reducing sodium intake and staying physically active address the fluid-retention side of the equation. For swelling caused by heart, kidney, or liver disease, treating the underlying condition is essential, as compression and elevation alone won’t overcome the systemic forces driving fluid into the tissues.