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    Blood clots: signs, symptoms and causes

    On this page
    1. What are blood clots?
    2. What causes blood clots to form?
    3. How do I know if I’m at risk of blood clots?
    4. Does the COVID-19 vaccine cause blood clots?
    5. What are the symptoms of a blood clot?
    6. How can I prevent blood clots?

    Reviewed by our clinical team

    Red blood cells

    Hearing the words “blood clot” can be scary – especially if you aren’t sure what they are or how they’re formed. The good news is that blood clots are rare, particularly in people who are young and healthy. To find out more, read on for a simple guide.

    What are blood clots?

    Clotting is a natural process in the human body where the blood thickens into a semi-solid state

    This happens in response to injury – clots form around the damaged area of the blood vessel to seal it up and stop further bleeding. Without the ability to form blood clots, we’d bleed excessively from the most minor injuries (in fact, this condition exists and is known as haemophilia).

    Most of the time, clotting happens as a safe, natural, and protective response to injury. However, sometimes blood clots can form in the blood vessels without an injury. When they fail to dissolve this can lead to dangerous blockages.

    A blood clot that forms in a vein, usually in the lower leg, is known as deep vein thrombosis (DVT). This condition can be really dangerous if the clot travels to the lungs and causes blood clot in the lung, a so-called "pulmonary embolism. A blood clot that forms in an artery in the brain, neck or heart can cause a stroke or a heart attack.

    What causes blood clots to form?

    There are a few different reasons why blood clots might form, including immobility.

    Immobility (i.e. not moving around) slows down blood flow, making clots more likely to form. Wheelchair users, people who have had surgery or in intensive care, or who have a leg in a cast, are more at risk of blood clots because they can’t move around. There’s also an increased risk of blood clots associated with travelling long distances by plane, train, coach or car.

    There are some medical conditions that can cause blood clots such as high cholesterol or being overweight.

    Additionally, some routine medications and medical treatments carry a slightly higher risk of developing blood clots e.g. chemotherapy, combined hormonal contraceptives, and hormone replacement therapy (HRT).

    How do I know if I’m at risk of blood clots?

    You may be more at risk of blood clots if you fall into any of the following categories:

    • You’ve recently had an operation 
    • You’re overweight 
    • You’re a smoker 
    • You’ve had a blood clot before 
    • You’re pregnant or you’ve recently had a baby 
    • You use combined pill, patch or ring or HRT 
    • You have an inflammatory condition like rheumatoid arthritis or Crohn’s disease

    None of these things mean that you’ll definitely get a blood clot, it just means that your risk is increased. Where possible, your healthcare professionals will take preventative steps to avoid increasing your risk further – for instance, your GP won’t prescribe the combined pill if you already have an increased risk of blood clots.

    Does the COVID-19 vaccine cause blood clots?

    There have been many reports about how one of the COVID-19 vaccines can cause blood clots. While this is worrying to hear, it’s important to know that the risk is extremely low: studies suggest that only four people in one million will develop blood clots after receiving the AstraZeneca vaccine.

    The blood clots caused by the AstraZeneca tend to happen in the brain. If you get the following symptoms 4-28 days after the AstraZeneca vaccine you must seek medical advice immediately:

    • a new, severe headache which is not helped by usual painkillers or is getting worse 
    • a headache which seems worse when lying down or bending over 
    • an unusual headache that may be accompanied by: 
    • blurred vision, nausea and vomiting 
    • difficulty with your speech 
    • weakness, drowsiness or seizures 
    • new, unexplained pinprick bruising or bleeding 
    • shortness of breath, chest pain, leg swelling or persistent abdominal pain.

    It’s also worth noting that the risk of a having a blood clot as a result of COVID-19 is around 10 times higher than the risk of having one after receiving the vaccine.

    What are the symptoms of a blood clot?

    According to the NHS, you should call 111 for advice immediately if you notice the following symptoms, as they may indicate a blood clot:

    1. Throbbing or cramping pain in the leg or arm 
    2. Swelling, redness, and warmth in the leg or arm 
    3. Sudden breathlessness and sharp pain in the chest, as well a cough that may bring up blood

    If someone is struggling to breathe or has passed out you should call 999 or go to A&E immediately.

    How can I prevent blood clots?

    If you’re under medical care – e.g. you’re in hospital for an operation – your doctors will take steps to prevent blood clots if they think you’re at risk. They might organise for you to have daily injections to thin your blood.

    Outside of a medical environment, there are a few lifestyle changes you can make to reduce your risk:

    • Avoid sitting for long periods and have regular mini breaks to get up and stretch your legs or walk around 
    • Stay hydrated by drinking plenty of water 
    • If you’re overweight, try to lose some weight 
    • Quit smoking 
    • Exercise more 
    • On long journeys where you have to stay sitting, wear compression socks, wriggle your toes and lower legs as much as possible and keep hydrated.


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