A blood clot forms when blood changes from a liquid into a gel-like mass. Clotting is essential to stop bleeding after an injury, but a clot that forms inside a blood vessel can block circulation. The symptoms and danger depend on whether the clot is in a vein or an artery and which organ is affected.
Types of harmful blood clot
- Deep vein thrombosis is a clot in a deep vein, usually in the leg
- Pulmonary embolism occurs when a clot travels to the lungs
- An arterial clot can interrupt blood flow to the heart, brain or a limb
Warning symptoms
A clot in a leg or arm may cause pain, swelling, warmth and a change in skin colour, usually on one side. A pulmonary embolism may cause sudden shortness of breath, chest pain that may worsen when breathing, coughing up blood, fainting or a very fast heartbeat.
A clot affecting the brain can cause sudden facial drooping, weakness or numbness on one side, speech difficulty, confusion, vision loss or loss of balance. A heart attack may cause pressure, tightness or pain in the chest, sometimes spreading to the arm, jaw, back or stomach.
Risk factors
Risk can increase after major surgery, hospital admission, prolonged immobility, serious injury or a previous clot. Other factors include cancer, pregnancy and the weeks after birth, oestrogen-containing medicines, inherited clotting conditions, smoking, older age and some long journeys. Clots can also occur without an obvious cause.
Diagnosis and treatment
Tests depend on the suspected location and can include ultrasound, blood tests, CT imaging or heart tests. Treatment commonly uses anticoagulant medicine to prevent the clot enlarging and reduce new clots. Some emergencies require medicine or a procedure to dissolve or remove the clot. Anticoagulants can cause bleeding and must be taken exactly as prescribed.
When to seek help
Get urgent medical assessment for new one-sided limb swelling and pain. Call emergency services immediately for sudden breathlessness, chest pain, coughing blood, fainting, stroke symptoms or a suddenly cold, pale and painful limb.
Sources
Reviewed: 28 August 2026