Deep vein thrombosis (DVT) is a blood clot in the deep veins of the leg, causing swelling and pain. If it is not treated, part of the clot can travel to the lungs and cause a pulmonary embolism, which can be fatal. Early diagnosis and the right treatment prevent these complications.
Medically reviewed by Dr. Aniruddha Bhuiyan, MCh vascular surgeon. Updated October 2026.
Blood clots in the veins can be dangerous, especially a DVT, which forms deep inside the body and can go unnoticed. Some clots cause only mild symptoms. Others lead to permanent vein damage, ulcers that will not heal, or a life-threatening pulmonary embolism.
Deep vein thrombosis (DVT) is a blood clot in one of the deep veins of the body, most often in the legs or pelvis. These veins carry used blood back to the heart. When a clot blocks a deep vein, blood cannot flow back properly, and the limb becomes painful, swollen and heavy. The NHS also explains DVT symptoms and treatment in plain language.
Without treatment, the clot can break free and travel through the bloodstream to the lungs. This is a pulmonary embolism, and it is a medical emergency.
DVT usually happens when three factors come together. Doctors call them Virchow’s triad:
Common triggers include:
DVT can be silent. Common signs are:
Other conditions can cause the same symptoms, so see a doctor promptly.
A pulmonary embolism happens when a piece of a DVT clot breaks off, travels to the lungs and blocks their blood supply.
It is a life-threatening emergency.
A large PE can cause sudden collapse or death without immediate treatment.
DVT affects the deep veins. Superficial venous thrombosis (SVT) affects veins close to the skin, usually visible, tender varicose veins.
SVT causes pain, redness and swelling along a vein under the skin. It is less dangerous than DVT, but it can spread into the deep veins, especially where the two systems join, so do not ignore it.
| Feature | SVT | DVT |
|---|---|---|
| Location | Just under the skin | Deep within the limb |
| Risk of PE | Low, but possible | High |
| Symptoms | Redness, cord-like vein | Swelling, deep pain |
| Treatment | Anti-inflammatory, sometimes anticoagulants | Anticoagulants, imaging, intervention |
A DVT can damage the valves in the veins. Blood then leaks backwards down the leg (venous reflux), which leads to:
This is why a DVT needs early diagnosis and proper treatment.
Treatment depends on where the clot is, how large it is, how long it has been there and whether there are complications. The aims are to:
See a vascular specialist if you:
Treating DVT early prevents long-term complications.
Swelling in one leg with pain, warmth or redness, especially after a long journey, surgery or bed rest, should be checked with a venous Doppler ultrasound. The scan is painless and shows whether there is a clot.
It needs prompt treatment, because part of the clot can travel to the lungs. Sudden breathlessness, chest pain or coughing up blood can mean a pulmonary embolism and need emergency care.
Usually 3 to 6 months. Some patients need longer or lifelong treatment, depending on what caused the clot and the risk of another.
Yes. Damage to the vein valves can cause post-thrombotic syndrome, with lasting swelling, darkening of the skin and ulcers near the ankle. Early treatment lowers this risk.
A DVT is a blood clot in a deep vein, usually in the leg. A PE (pulmonary embolism) happens when part of that clot breaks off and travels to the lungs. A DVT causes leg swelling and pain; a PE causes sudden breathlessness, chest pain or coughing up blood, and is an emergency.
Many people ignore leg pain or swelling until it becomes serious. DVT and PE can be prevented, and they can be treated well when caught early. We aim to find vein disease early, treat it properly and follow patients over the long term so it does not come back. If you are at risk or have symptoms, book a consultation today.
