Legs and feet swell when fluid leaks out of tiny blood vessels and gets trapped in the surrounding tissue. This happens because of a pressure imbalance: your capillaries are either pushing too much fluid out, or not pulling enough of it back in. Gravity makes the lower body the most common place for this fluid to collect, especially if you’ve been standing or sitting for long stretches.
How Fluid Ends Up Trapped in Your Tissues
Your body constantly moves fluid between your bloodstream and the tissues around it. Two opposing forces keep this exchange in balance. Pressure inside your blood vessels pushes fluid outward, while proteins (mainly one called albumin) act like a sponge, pulling fluid back in. When either side of this equation shifts, fluid accumulates in the spaces between your cells. That visible puffiness in your ankles or feet is the result.
Several things can tip this balance. Anything that raises pressure inside your veins, like blood pooling in your legs, forces more fluid out. Anything that lowers the protein concentration in your blood, like liver or kidney problems, weakens the pull that brings fluid back. And damage to the vessel walls themselves can make them leakier. The medical term for this swelling is edema, and the lower legs are its favorite target because gravity keeps fluid from draining back upward easily.
Vein Problems Are the Most Common Cause
Your leg veins contain one-way valves that push blood upward toward your heart, working against gravity with every step you take. When those valves weaken or become damaged, blood flows backward and pools in the lower legs. This condition, called chronic venous insufficiency, is one of the most frequent reasons for persistent leg swelling. The pooling blood raises pressure in those veins, which forces fluid out into the surrounding tissue.
Swelling from venous insufficiency tends to worsen as the day goes on and improves overnight when your legs are level with your heart. You might also notice visible varicose veins, brownish skin discoloration around the ankles, or dry, itchy patches on the lower legs. In more advanced cases, the skin near the inner ankle can break down into slow-healing ulcers.
Heart, Kidney, and Liver Disease
When the heart can’t pump blood efficiently, the reduced circulation triggers a chain reaction. Your body senses that organs aren’t getting enough blood and responds by holding onto sodium and water to try to increase blood volume. But since the heart is already struggling, this extra fluid just backs up into the veins and eventually leaks into the tissues. The result is swelling in the feet, ankles, and lower legs, often accompanied by shortness of breath or fatigue.
Kidney disease creates a similar problem from the other direction. When the kidneys can’t filter and excrete sodium properly, fluid volume rises throughout the body. The extra fluid has to go somewhere, and gravity pulls it downward. Liver disease contributes differently: a failing liver produces less albumin, the protein responsible for pulling fluid back into the bloodstream. With less albumin circulating, fluid seeps into tissues more readily.
Swelling caused by these organ-level problems tends to affect both legs equally. If you press a finger into the swollen area and it leaves a visible dent that takes several seconds to fill back in, that’s called pitting edema, and it’s characteristic of these systemic causes.
Medications That Cause Swelling
Several widely prescribed medications can trigger leg and foot swelling as a side effect. Blood pressure drugs in the calcium channel blocker family are among the most common culprits. They work by relaxing blood vessel walls, which can widen the small arteries more than the veins, increasing the pressure difference that pushes fluid out of capillaries.
Other medications linked to swelling include:
- Anti-inflammatory painkillers (NSAIDs) like ibuprofen and naproxen, which cause the kidneys to retain sodium
- Nerve pain medications such as gabapentin and pregabalin
- Steroids like prednisone, which promote salt and water retention
- Diabetes medications in the thiazolidinedione class
- Dopamine agonists used for Parkinson’s disease and restless legs
- Insulin, particularly when starting treatment or adjusting doses
If swelling appeared or worsened after starting a new medication, that connection is worth raising with your prescriber. Stopping or switching the drug often resolves the problem, though you should never discontinue a prescribed medication on your own.
Pregnancy and Hormonal Changes
Swollen feet and ankles are one of the most common complaints during pregnancy, particularly in the third trimester. The reasons are layered. Total body water increases by 6 to 8 liters during pregnancy, with about two-thirds of that extra fluid sitting outside the bloodstream. The growing uterus also compresses the veins and lymphatic channels that drain the lower body, slowing the return of blood and fluid back up toward the heart.
Hormonal shifts further relax blood vessel walls, and the cardiovascular system undergoes significant remodeling to support the placenta. All of this increases the amount of blood flowing to the lower half of the body. Pregnancy also raises the risk of developing varicose veins in the legs and pelvis, which can persist after delivery. Mild, symmetrical swelling that worsens with standing is normal in pregnancy. Sudden, severe swelling, especially if paired with high blood pressure or headaches, is a different situation entirely and needs immediate evaluation.
Salt, Sitting, and Other Everyday Triggers
A high-sodium meal can cause noticeable swelling within hours. When you eat more salt than your body needs, the kidneys hold onto extra water to keep the sodium concentration in your blood from rising too high. This temporarily expands the volume of fluid in your body, and gravity pulls the excess into your lower extremities. The effect is usually mild and resolves as your kidneys catch up over the next day or two.
Prolonged sitting or standing has a similar effect through a different mechanism. Without the pumping action of your calf muscles, blood pools in the lower legs and venous pressure climbs. Long flights are a classic trigger. Heat also plays a role: warm temperatures dilate blood vessels near the skin’s surface, which can increase fluid leakage into the tissues. These lifestyle triggers typically cause symmetrical swelling in both legs that improves with elevation and movement.
One Leg vs. Both Legs: What the Pattern Tells You
Whether the swelling affects one leg or both is one of the most useful clues for figuring out the cause. Symmetrical swelling in both legs usually points to a systemic issue: heart failure, kidney disease, medication side effects, or simply too much time on your feet. The problem is body-wide, and both legs are affected roughly equally.
Swelling in just one leg raises different concerns. The two primary causes of unilateral swelling are deep vein thrombosis (a blood clot) and lymphedema (a failure of the lymph drainage system). A blood clot typically comes on relatively quickly and may be accompanied by calf pain or tenderness, warmth, redness, and visible surface veins. The swelling pits when you press on it. If the clot is below the knee, the swelling stays in the calf and foot. If it’s higher up, swelling can extend to the thigh.
Lymphedema looks and feels different. It tends to develop gradually and characteristically involves the top of the foot and toes, giving them a squared-off, boxy appearance. In early stages the swelling may pit, but over time the tissue becomes firm and you can no longer press a dent into it. A useful test: try to pinch a fold of skin on the top of the foot near the base of the second toe. If the skin is too thick to pinch, that strongly suggests lymphedema. In advanced cases the skin becomes rough, thickened, and develops a cobblestone-like texture.
Managing and Reducing Swelling
The most effective treatment depends on the underlying cause, but several strategies help across the board. Elevating your legs above heart level for 20 to 30 minutes a few times a day lets gravity work in your favor, draining fluid back toward your core. Regular movement, even short walks or calf raises, activates the muscle pump that pushes blood upward through your veins.
Compression stockings are one of the most reliable tools for managing chronic swelling. They come in different pressure levels measured in millimeters of mercury (mmHg). For mild swelling or early vein problems, low-pressure stockings in the 15 to 21 mmHg range can prevent fluid from accumulating. For established venous edema, higher pressures around 40 mmHg are more effective at actively reducing swelling. Severe venous disease and active ulcers may call for 40 to 50 mmHg, while lymphedema often requires even higher pressures during the initial reduction phase before stepping down to a lighter maintenance garment. Higher-pressure stockings are harder to get on and less comfortable, so compliance tends to drop. Starting at the lowest effective pressure is practical advice.
Reducing sodium intake helps if fluid retention is part of the picture. Processed and restaurant foods account for most dietary sodium, so cooking at home gives you more control. Staying physically active, maintaining a healthy weight, and avoiding long stretches of sitting or standing without breaks all reduce the gravitational load on your lower-leg veins. For swelling caused by organ disease, treating the underlying condition is essential, as compression and elevation alone won’t resolve the root problem.