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What to do if…
one leg suddenly becomes swollen, painful, or warm and you fear a blood clot

Produced and maintained by PanicStation.org Published: Last reviewed: Editorial policy UK guide

Short answer

Get urgent medical help now because a suspected deep vein thrombosis needs prompt assessment. Ask for an urgent GP appointment or contact NHS 111 in England, Scotland or Wales; in Northern Ireland, contact your GP, GP out-of-hours service or local Phone First service.

Do not do these things

  • Do not wait until tomorrow to seek advice about sudden one-sided leg swelling or pain.
  • Do not assume it is only a cramp, strain, infection or fluid retention without assessment.
  • Do not start aspirin, anticoagulants or someone else’s medicine unless a clinician tells you to.
  • Do not change or stop prescribed medicines unless a clinician advises you.
  • Do not drive yourself to A&E if you have chest pain, severe breathing difficulty, a very fast heartbeat or feel faint.

What to do now

  1. Ask for an urgent GP appointment or contact the appropriate urgent service now. Use NHS 111 in England, Scotland or Wales. In Northern Ireland, contact your GP, GP out-of-hours service or local Phone First service.
  2. Say clearly that one leg has suddenly become swollen, painful, warm, red, blue or darker and that you are concerned about a possible deep vein thrombosis.
  3. Call 999 or go to A&E immediately if you have leg pain or swelling together with shortness of breath or chest pain.
  4. Call 999 immediately if you have severe difficulty breathing, chest or upper-back pain, a very fast heartbeat, or you pass out. Do not drive yourself.
  5. Note when the leg symptoms began and whether they affect the calf, thigh or whole leg.
  6. Tell the clinician about any recent operation, hospital stay, reduced mobility, long journey, pregnancy, recent birth, cancer, hormone replacement therapy, oestrogen-containing contraception or previous blood clot.
  7. Bring your medicines or a current medication list if you are asked to attend for assessment.

What can wait

You do not need to work out the cause, compare treatments, buy compression stockings or make long-term medication decisions now. Focus on getting urgent assessment and watching for breathing difficulty, chest pain, a very fast heartbeat or fainting.

Important reassurance

A swollen or painful leg can have several causes, so these symptoms do not confirm a blood clot. Prompt assessment matters because symptoms alone cannot safely establish the cause and a clot can be serious.

Scope note

This guide covers immediate first steps only. Testing, diagnosis, treatment and later prevention decisions may require assessment by a GP, urgent-care clinician or hospital team.

Important note

This is general health information, not a diagnosis or a substitute for urgent medical assessment. Call 999 for symptoms that may be immediately life-threatening.

Additional Resources

Closely connected guides that may be useful during the same situation or its immediate aftermath.

About this guide

This guide was produced and is maintained by PanicStation.org using its published editorial process. Official and specialist sources are checked where relevant, and AI-assisted tools may be used for drafting, organisation, and consistency checks. The site operator remains responsible for publication, revision, and removal decisions.

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