Your feet swell when fluid leaks out of tiny blood vessels and gets trapped in the surrounding tissue. This is one of the most common physical complaints, and it ranges from completely harmless (you stood all day or ate a salty meal) to a signal that your heart, kidneys, or veins aren’t working properly. The cause usually comes down to one of a few basic mechanisms, and the pattern of swelling, whether it’s in one foot or both, tells you a lot about what’s going on.
How Fluid Ends Up in Your Feet
Your body constantly moves fluid between your bloodstream and the tissues around it. Two opposing forces control this exchange: the pressure of blood pushing fluid outward through capillary walls, and the pull of proteins in your blood drawing fluid back in. When these forces fall out of balance, fluid escapes into tissue faster than it can be reabsorbed, and gravity pulls that excess fluid straight down to your feet and ankles.
Five things can tip this balance: increased blood pressure inside your capillaries, lower protein levels in your blood (which weakens the pull that draws fluid back in), problems with your lymphatic drainage system, changes in tissue pressure, or a combination of several at once. Nearly every cause of foot swelling traces back to one of these mechanisms.
Everyday Causes That Are Usually Harmless
The most common reason for swollen feet is simply spending too long in one position. When you sit at a desk for hours or stand on your feet all day, blood pools in the veins of your lower legs. That pooling raises the pressure inside those veins, which gets transmitted backward into the capillaries, forcing more fluid out into the tissue. Long flights are a classic trigger for the same reason.
Eating a lot of salty food causes your body to hold onto extra water, which increases the overall volume of fluid in your bloodstream and raises pressure in your capillaries. Hormonal shifts before your period do something similar. Pregnancy causes swelling in most women, partly from increased blood volume and partly from the weight of the uterus compressing veins that return blood from the legs. In all of these cases, the swelling is typically mild, affects both feet equally, and goes down after you rest with your feet elevated.
Medications That Cause Swelling
Several common medications list foot and ankle swelling as a side effect. Calcium channel blockers, a widely prescribed class of blood pressure medication, are one of the most frequent culprits. Swelling occurs in roughly 1 to 15% of people taking standard doses of these drugs, and at high doses over long periods, the rate can exceed 80%. The swelling happens because these medications relax blood vessel walls, which increases blood flow and pressure in the capillaries of the lower legs.
Other medications that commonly cause swelling include anti-inflammatory painkillers (like ibuprofen and naproxen), steroid medications, estrogen-based hormone therapies, and certain diabetes drugs. If your feet started swelling around the time you began a new medication, that connection is worth raising with whoever prescribed it.
Venous Insufficiency: When Vein Valves Wear Out
Your leg veins contain a series of one-way valves that open to let blood flow upward toward your heart and close to prevent it from falling back down. Over time, these valves can weaken or become damaged, allowing blood to flow backward and pool in your lower legs. This condition, called chronic venous insufficiency, is extremely common. Varicose veins affect more than 25 million adults in the United States alone, and more advanced venous disease affects over 6 million.
Prevalence rises sharply with age. Studies using ultrasound screening have found venous insufficiency in about 21% of men and 12% of women over age 50. Previous blood clots in the deep veins are a major cause of valve damage, but valves also deteriorate simply from years of standing and the constant downward pull of gravity. The swelling from venous insufficiency tends to worsen throughout the day, improve overnight, and affect both legs, though not always equally. You may also notice skin discoloration, a heavy or achy feeling in your legs, or visible varicose veins.
Heart, Kidney, and Liver Problems
When foot swelling is caused by a problem with a major organ, it almost always affects both feet and often comes with other symptoms.
In heart failure, the heart can’t pump blood efficiently, so blood backs up in the veins. That backup raises pressure in the capillaries of your legs and feet, pushing fluid into the tissue. The swelling often extends up the ankles and shins, and you may notice it worsening over the course of the day. Shortness of breath, especially when lying flat, is a key accompanying symptom because the same backup of fluid can affect the lungs.
Kidney disease causes swelling through a different route. When the kidneys can’t properly filter waste and regulate fluid balance, salt and water build up in the bloodstream. Kidney-related swelling often shows up in the legs but also around the eyes, which is a distinguishing clue. In nephrotic syndrome, a specific type of kidney damage, the kidneys leak protein into the urine. That loss of protein lowers the pulling force that keeps fluid inside your blood vessels, and swelling results.
Liver damage from cirrhosis reduces the liver’s ability to produce albumin, the main protein responsible for keeping fluid in your bloodstream. Low albumin levels mean less pull to reabsorb fluid, so it accumulates in the legs and abdomen.
One Swollen Foot Is Different From Two
Pay close attention to whether the swelling is in one foot or both. Swelling in both feet usually points to a systemic cause: too much salt, a medication side effect, venous insufficiency, or an organ-related issue. Swelling in just one foot or leg raises a different set of concerns.
The most urgent possibility with one-sided swelling is a deep vein thrombosis (DVT), a blood clot in one of the deep veins of the leg. DVT typically causes swelling, pain, warmth, and sometimes redness in the affected leg. It’s more likely after long periods of immobility, such as a long flight or recovery from surgery. A DVT requires prompt medical attention because the clot can break loose and travel to the lungs.
One-sided swelling can also result from a local injury, infection, or lymphedema. Lymphedema occurs when the lymphatic system, which normally drains excess fluid from tissues, is blocked or damaged. Unlike regular swelling, which involves low-protein fluid, lymphedema produces a protein-rich swelling that tends to be firmer and doesn’t resolve as easily with elevation. It can develop after surgery or radiation that damages lymph nodes, or it can occur without an obvious cause.
Pregnancy Swelling vs. Preeclampsia
Some degree of foot swelling during pregnancy is normal, especially in the third trimester. But sudden or severe swelling, particularly in the hands and face rather than just the feet, can be a warning sign of preeclampsia. This condition is diagnosed when blood pressure rises to 140/90 mmHg or higher after the 20th week of pregnancy, along with protein in the urine. Preeclampsia is a serious condition that requires medical management, so rapid swelling during pregnancy warrants prompt evaluation.
What Helps Reduce the Swelling
For swelling caused by everyday factors like prolonged sitting, gravity, or salt intake, the most effective immediate step is elevating your feet above the level of your heart while sitting or lying down. This reverses the gravitational pressure that drives fluid into your feet and lets it drain back toward your core. The higher you can comfortably get your feet, and the longer you keep them there, the more fluid will move out.
Reducing sodium in your diet makes a meaningful difference for people whose swelling is fluid-retention related. Movement is equally important. Walking activates the calf muscles, which act as a pump that squeezes blood upward through your veins. Even short walks or calf raises throughout the day can prevent the blood pooling that leads to swelling.
Compression stockings apply steady external pressure to your legs, counteracting the forces that push fluid out of your capillaries. They come in different pressure levels: 15 to 20 mmHg provides mild support for early or occasional swelling, while 30 to 40 mmHg is typically used for chronic venous insufficiency or lymphedema affecting the lower legs. Higher pressures (40 to 50 mmHg and above) are reserved for severe cases with significant tissue changes and should only be used after clinical assessment.
Warning Signs That Need Prompt Attention
Most foot swelling is benign and temporary. But certain patterns warrant urgent evaluation. Swelling accompanied by shortness of breath, chest pain, or an irregular heartbeat can indicate fluid buildup in the lungs and needs immediate attention. Swelling in one leg that comes with pain, warmth, or redness, especially after a period of immobility, suggests a possible blood clot. And swelling that develops rapidly, doesn’t improve with elevation, or keeps getting worse over days to weeks may point to an underlying organ problem that needs investigation.