What would cause only one foot to swell? A comprehensive guide

September 25, 2025 •

4 min read

According to the National Institutes of Health, localized swelling, or edema, in a single limb is often caused by a regional issue, unlike the bilateral swelling seen with systemic conditions. This guide explores what would cause only one foot to swell? and why medical evaluation is so important.

Quick Summary

A swollen foot occurring on only one side is typically caused by a localized problem such as an injury, a serious blood clot (DVT), a skin infection (cellulitis), chronic venous insufficiency, or a lymphatic blockage, requiring prompt diagnosis.

Key Points

DVT is an emergency: Sudden, unilateral swelling with pain, warmth, or redness could signal a deep vein thrombosis, a medical emergency requiring immediate attention.
Check for injury: A sprain, fracture, or muscle strain from an identifiable event is a very common cause of swelling in one foot.
Watch for infection: A rapidly spreading red, warm, and painful area, especially with fever, suggests cellulitis, a serious bacterial infection.
Consider chronic issues: Gradual, persistent swelling could be related to long-term venous insufficiency or lymphedema, which require ongoing management.
Know the difference: Swelling in one foot is distinct from bilateral swelling; it points to a localized problem in that leg rather than a systemic organ issue.

Understanding Unilateral Foot Swelling

Swelling in the feet and ankles, known as edema, is a common symptom with many potential causes. When the swelling is localized to only one foot, it often points to a specific issue in that leg, foot, or ankle, rather than a whole-body (systemic) problem like heart or kidney disease. While some causes are minor, others can be severe and require immediate medical attention. Distinguishing between these possibilities is the first step toward finding relief and ensuring safety.

Life-Threatening Conditions Requiring Immediate Care

Some of the most critical causes of a single swollen foot are related to blood circulation and infection. Recognizing these signs quickly can be life-saving.

Deep Vein Thrombosis (DVT)

This is perhaps the most serious cause of unilateral leg and foot swelling. A DVT is a blood clot that forms in a deep vein, most often in the lower leg or thigh. If the clot breaks loose and travels to the lungs, it can cause a pulmonary embolism, which is a medical emergency. Swelling from a DVT typically occurs suddenly and may be accompanied by:

Pain or tenderness, often starting in the calf and feeling like a cramp
Warmth in the affected area
Red or discolored skin
A heavy or aching sensation

Infection (Cellulitis)

Cellulitis is a serious bacterial skin infection that can also cause significant swelling. It often develops after a break in the skin, such as a cut, insect bite, or puncture wound, allows bacteria to enter. Symptoms include:

Redness and swelling that spreads rapidly
Warmth and tenderness to the touch
Pain in the affected area
Fever, chills, and fatigue

Common Injuries and Traumas

Direct trauma to the foot or ankle is a frequent and straightforward cause of localized swelling. The body's inflammatory response rushes blood and fluid to the site of the injury to begin the healing process.

Sprains and fractures: Ankle sprains or foot fractures, even hairline (stress) fractures, can cause significant swelling, pain, and bruising.
Bone bruises: An impact or trauma can cause a bone bruise, leading to swelling and pain that may last longer than a typical bruise.
Tendonitis: Inflammation of a tendon, such as the Achilles tendon, can cause swelling near the heel or ankle.

Chronic and Less Urgent Causes

When swelling develops more gradually or persists over time, it may be due to a chronic underlying condition.

Chronic Venous Insufficiency (CVI): This condition occurs when the valves in the leg veins are damaged, causing blood to pool instead of flowing back to the heart. Over time, this pressure can cause fluid to leak into the surrounding tissue, leading to unilateral swelling, varicose veins, and skin changes.
Lymphedema: A blockage or damage to the lymphatic system can cause lymphatic fluid to accumulate in the tissues. This can be a side effect of surgery (e.g., cancer treatment involving lymph node removal), infection, or, in rare cases, a congenital issue.
Arthritis: Certain forms of arthritis can affect a single joint in the foot, causing localized pain and swelling. Gout, which results from a buildup of uric acid crystals, commonly affects the big toe.

Other Potential Contributors

Allergic reaction: An insect bite or sting on the foot can cause a localized allergic reaction with swelling, redness, and itching.
Cysts or tumors: In rare cases, a benign or malignant mass can press on blood vessels or lymphatic pathways, causing unilateral swelling.
Prolonged standing or sitting: While this typically causes bilateral swelling, in a leg with poor circulation, the effect may be more pronounced on one side.

Comparing Causes of Unilateral Foot Swelling

To better understand the differences between potential causes, consider the following comparison of symptoms and characteristics. This table is for informational purposes only and is not a substitute for a medical diagnosis.


Feature	DVT (Blood Clot)	Cellulitis (Infection)	Injury (Sprain/Fracture)	Chronic Venous Insufficiency
Onset	Sudden, often without clear cause	Can be sudden after an injury	Sudden after a specific event	Gradual, chronic
Pain	Aching, cramping, tenderness	Tenderness, throbbing	Sharp or aching, dependent on injury	Aching, heaviness, often worse later in day
Skin	Red or discolored, warm	Spreading redness, warm, sometimes fever	Bruising, swelling, possibly deformity	Skin discoloration, thickening, varicose veins
Urgency	Medical Emergency	High Urgency	Varies, potentially urgent if severe	Can be managed, but requires evaluation

Conclusion: Seeking Medical Evaluation

Because the causes of a swollen foot on only one side range from minor irritations to severe medical emergencies like DVT, it is always recommended to seek medical advice. A healthcare professional can perform a proper evaluation, which may include imaging like an ultrasound, to determine the underlying cause and ensure you receive the correct treatment. Ignoring a swollen foot can have serious consequences, so prompt attention is key to safeguarding your health.

For more information on Deep Vein Thrombosis, refer to the National Heart, Lung, and Blood Institute.

Frequently Asked Questions

While an injury like a sprain is a very common cause, other serious conditions like a Deep Vein Thrombosis (DVT) must be ruled out by a medical professional. An infection, like cellulitis, is also a frequent cause.

You should see a doctor immediately if the swelling occurs suddenly, is painful, or is accompanied by redness, warmth, or a fever. You should also seek medical advice if the swelling does not improve with at-home care or if you have a history of blood clots.

Yes, a severe ankle sprain or foot fracture can cause swelling that spreads throughout the entire foot as the body’s inflammatory response sends fluid and immune cells to the injured area.

While standing or sitting for prolonged periods typically causes swelling in both feet, it can be more prominent in one if that leg has underlying circulatory issues, such as venous insufficiency.

Doctors will typically conduct a physical examination and take a full medical history. To check for a DVT, they may order a vascular ultrasound. Other tests, such as X-rays for fractures or blood tests for infection or gout, may also be used.

DVT is a specific, often acute, blood clot in a deep vein, which is a medical emergency. Chronic venous insufficiency is a long-term problem with vein valves that leads to chronic pooling of blood and swelling, developing gradually over time.

Most medications that cause swelling (edema) lead to bilateral swelling. However, if an individual has a pre-existing condition affecting one leg, the fluid retention from medication could cause more noticeable swelling on that side.