DVT (deep vein thrombosis)

Animation - how deep vein thrombosis forms

About deep vein thrombosis

Deep veins pass through the centre of your leg and are surrounded by layers of muscle. DVT happens when a blood clot forms in a deep vein.

It affects about one in 1,000 people every year in the UK. DVT is most common in the deep veins of your lower leg (calf). It can also develop in the deep veins in your thigh and, more rarely, in other deep veins, such as the ones in your arm or pelvis.

Sometimes blood clots can form in your superficial veins, which lie just under your skin. This is known as superficial thrombophlebitis. The condition is different to DVT and much less serious, but it can occasionally spread to your deep veins.

Symptoms of deep vein thrombosis

Many blood clots are small and don't cause any symptoms. Your body will often be able to gradually break down a blood clot with no long-term effects.

Larger clots can partly or completely block the blood flow in your vein and cause symptoms in your affected leg or arm, such as:

  • swelling
  • pain and tenderness – this may be along the path of a vein and you may find it difficult to stand with your full weight on the affected leg
  • skin that feels warm or hot to the touch and looks red
  • a mild fever

Although these symptoms may not be caused by DVT, if you have them, you should see your GP urgently for advice.

Complications of deep vein thrombosis

DVT may not cause you any further problems, but possible complications can include the following.

Pulmonary embolism

This is the most serious complication of DVT. A pulmonary embolism happens when the blood clot (or a piece of it that has broken off) travels in your blood to your lungs, where it blocks one of your blood vessels. This is serious and can be fatal.

Post-thrombotic syndrome

This happens if DVT damages the valves in your veins and permanently reduces the ability of your veins to efficiently return blood from your lower leg to your heart. This can lead to ongoing high pressure in the veins in your legs and can eventually cause long-term pain and swelling. If the condition becomes severe, you may develop ulcers on your leg.

Limb ischaemia

This is a rare complication that only happens with severe DVT. The blood clot can cause the pressure in your vein to rise. This can block the flow of blood through your arteries, so less oxygen is carried to your affected leg. This can be painful and lead to skin ulcers, infection and even gangrene.

Causes of deep vein thrombosis

Anyone can get DVT but many things may increase your risk of the condition. Often, more than one factor will contribute to its development – some of these include:

  • your age – DVT is more likely as you get older
  • smoking
  • immobility – for example, if you have had an operation (especially on your hip or knee), are bed bound or are travelling a long distance and aren’t able to move your legs
  • having had DVT before
  • someone else in your family having had DVT
  • thrombophilia – this is an inherited condition that causes your blood to clot more easily
  • being overweight or obese
  • having certain long-term conditions, such as cancer and some inflammatory conditions
  • antiphospholipid syndrome – this is a condition in which your blood is at a greater risk of clotting than usual
  • taking a contraceptive pill that contains oestrogen, or taking hormone replacement therapy (HRT)
  • pregnancy or having recently had a baby

Travel risks

There is evidence that long-haul travel (lasting over four hours) can increase your risk of developing DVT. Although you usually hear about DVT in relation to flying, this is confusing because the risk is mainly the result of being inactive for long periods of time. This can happen during any form of long-distance travel, whether by car, bus, train or air. Therefore, you may hear the phrase ‘travel-related DVT’ being used.

Generally, your risk of developing DVT when travelling is small unless you have one or more of the risk factors mentioned previously. If you do, talk to your GP before you travel for four hours or more.

Diagnosis of deep vein thrombosis

Your GP will ask about your symptoms and examine you. If he or she thinks that you might have a DVT, you may be referred to a specialist in a hospital or clinic for further tests. These may include the following.

  • A blood test called a D-dimer. This measures a substance that develops when a blood clot breaks down. If this test is negative, it's unlikely that you have DVT.
  • Ultrasound. This uses sound waves to look at your blood as it flows through your blood vessels. It's the best test to find blood clots that are above your knee but is less effective at picking up blood clots in your lower leg.
  • Venography. A special dye, which shows up on X-ray, is injected into your vein and X-rays of your leg are taken. This is the best way of showing clots that are below your knee and may be carried out if an ultrasound doesn’t show anything abnormal but your doctor still suspects that you may have DVT.

Treatment of deep vein thrombosis

The main aim of treatment for DVT is to reduce the risk of and, if possible, prevent complications including pulmonary embolism. It will also try to relieve any symptoms you have, such as pain and swelling.

Self help

Try to start walking as soon as possible after DVT. This has been found to be safe and it may help circulation of blood in your leg. It may also help to ease your symptoms and reduce your risk of developing complications.


You will be given anticlotting (anticoagulant) medicines, such as low molecular weight heparin injections or warfarin tablets, to treat DVT. These change chemicals, known as clotting factors, in your blood and so reduce its ability to clot and can also stop new blood clots from forming. There are a number of new medicines available, for example rivaroxaban, that may be more suitable for some people. However, because they are new, there isn’t much evidence about their long-term effectiveness. You will probably need to take anticlotting medicines for at least three months and possibly longer.

Compression stockings

These are also called graduated compression stockings. Your doctor may recommend that you wear them as they may help to reduce pain and swelling. They can also help to prevent post-thrombotic syndrome. You will need to have these fitted for you by a nurse or pharmacist and may need to wear them for at least two years after developing DVT.

Inferior vena cava filters

If there is a reason why you can’t take anticlotting medicines, your doctor may recommend having a filter fitted in your inferior vena cava. This may be because you’re pregnant or you have recently had major surgery. Your vena cava is a large vein in your abdomen (tummy) and blood travels in it from your lower body back to your heart. Placing a filter in it means that if a blood clot (or a piece of it) moves from your leg, it will be prevented from travelling to your heart or lungs where it could cause pulmonary embolism.

Prevention of deep vein thrombosis

Ask your GP for advice if you think you're at risk of developing DVT.

There are things you can do to reduce your risk, such as stopping smoking if you smoke, or losing weight if you're overweight. Regular walking can help to improve the circulation in your legs and help to prevent another DVT from developing. If you have had a DVT, you can usually start walking once you get home from the hospital, unless your doctor has told you otherwise.

There isn't enough evidence to show that taking aspirin reduces your risk of developing DVT.

If you're having surgery

Surgery and some medical treatments can increase your risk of developing DVT. So if you're going to hospital for an operation, you will usually have an assessment to check your risk of developing DVT during surgery. There are many things that can be done to try to keep this risk as low as possible.

You may be given anticlotting medicines before and after surgery, or be asked to wear compression stockings. You may also be given a mechanical pump to use on your feet and legs in the first few days after the operation. This is called an intermittent compression device. The pump automatically squeezes your feet and lower legs to help your blood circulate.

If you're travelling

If you have recently been treated for DVT, you will need to wait two weeks before making journeys that will be longer than four hours.

Although it's unlikely that you will develop DVT when you're travelling, there are some steps you can take to reduce your risk of developing a blood clot on journeys of over three hours.

  • Take short walks – if you're a passenger, walk up and down the aisle of the train or plane.
  • Exercise the muscles of your lower legs as these act as a pump for the blood in your veins. Regularly bend and straighten your toes, ankles and legs.
  • Wear loose-fitting clothes.
  • Keep hydrated by making sure you drink enough water.
  • Don’t drink too much alcohol or caffeine.
  • Don’t take sleeping tablets – if you’re asleep throughout the journey, you won’t be able to move around and keep your legs active.
  • Wear compression stockings if you have other risk factors for DVT (for example, if you have had a previous travel-related or unprovoked DVT, recently had surgery or have cancer).

If your GP has told you that you're at high risk for DVT, you may also need low molecular weight heparin injections for long journeys. Talk to your GP for more information.

If you develop swelling or pain in your calf or thigh during or a few hours or days after your flight, or if you have breathing problems or chest pain after travelling, you should seek urgent medical attention.

If I have had a deep vein thrombosis, am I at a greater risk of getting another one?


About three in 10 people who have had a DVT or pulmonary embolism will have another one within 10 years of the first. This is most likely during the first two years afterwards, and your risk of developing DVT decreases over time. You’re also more at risk if your first DVT didn’t happen after surgery or an accident. You can reduce your risk of developing DVT again by continuing to take any medicines that your doctor prescribes for you and making any lifestyle changes that he or she advises.


Having a DVT increases your chance of having one again. You're more likely to develop a DVT again if the first one occurred without an obvious cause or you got it because of an underlying medical condition that hasn't gone away since then. You're less likely to have one again if you developed a DVT because of a one-off event, such as an operation.

If you develop DVT, your doctor will probably first treat you with injections of an anticlotting medicine called low molecular weight heparin followed by warfarin that you take by mouth. How long you need to take this medicine for will depend on what caused your DVT and how likely you are to develop it again. You may only need to take it for a few months, but it’s possible that you will need to continue taking this medicine for life. You may be advised to wear compression stockings for at least two years.

Because you have already had DVT, you will be more likely to develop it after long-distance travel than someone who has never had DVT. Therefore, it’s important to get medical advice before making any travel plans that involve long journeys.

I'm having an operation. How can I reduce my risk of deep vein thrombosis?


There are a number of things that can help to prevent DVT and your surgeon or nurse will be able to give you information about these. You can also reduce your risk of developing DVT by following your surgeon or nurse’s advice about treatment after surgery. This may include getting up and about as soon as possible after your operation and taking the medicines that have been prescribed for you.


Your risk of developing DVT after an operation depends on a number of things, including the type of surgery you’re having and whether you have any other risk factors. In addition, being inactive increases your risk of DVT. After an operation, it’s possible that you will need to spend a lot of time in bed and won’t be able to move around a great deal.

Your surgeon or nurse will assess you when you go into hospital for your operation and will develop a plan to help prevent DVT. This will include information about your risk of developing DVT, what you can do to reduce this and what you need to do once you go home. This is likely to include stopping smoking, wearing compression stockings, taking anticlotting medicines and doing foot and leg exercises when you're in bed or inactive.

Other ways to reduce your risk may include the following.

  • If you're taking the contraceptive pill, your surgeon may ask you to stop taking it and use other methods of contraception for the month before your operation.
  • You may be asked to wear compression stockings, before, during and after your operation until you're back to your usual levels of mobility and activity. These help to keep the blood flowing well through your legs by squeezing your muscles and prevent clots from forming. Sometimes these can be attached to a device that fills the stockings with air and then deflates them, helping to squeeze your leg muscles.
  • Your surgeon or nurse will encourage you to get up and move around as soon as you can after your operation. You will also be shown some foot and leg exercises to do while you're in bed or not very active. These increase the blood flow in your legs, which can help to prevent clots.
  • Make sure you drink enough fluid as soon as you're able to after your operation. Staying properly hydrated can help to prevent blood clots.
  • You may need to have injections of a medicine called low molecular weight heparin to help thin your blood. You will need to have these injections until you're up and about and back to your usual level of mobility, or maybe for longer, for example after a hip replacement. There are also new medicines that are available in tablet form, such as rivaroxaban, that you may be given as an alternative to injections. However, these aren't suitable for everyone.

If you don't get this information, it’s important that you ask for it.

Are there any risks associated with taking warfarin to treat deep vein thrombosis?


Warfarin is an anticlotting medicine that you may need to take for several months after DVT. Like all medicines, warfarin can have side-effects but you can reduce your risk of these by taking it every day as your doctor prescribes. You will also need to have regular blood tests.


Warfarin is the anticlotting medicine that you’re most likely to be prescribed after DVT. You take it as one or more tablets once a day and it's important that you take it at the same time every day. Like all medicines, warfarin can have side-effects, but you can reduce your risk of these by following the advice below.

  • Make sure you go for your blood tests. You will have regular blood tests when taking warfarin to monitor how well the medicine is working and to make sure you're taking the right amount. Too much can cause bleeding and too little can increase your risk of another clot forming. You will need blood tests every day or every other day when you first start taking the medicine. Depending on how well it works for you, you will then need to have these less often and eventually only every 12 weeks.
  • Carry an alert card or wear a medical bracelet that indicates that you’re taking warfarin. It's important that any health professional who treats you knows that you're taking an anticlotting medicine because it can affect your treatment and interfere with other medicines. This includes over-the-counter medicines that don't need a prescription and any complementary or herbal remedies. Tell any doctor, nurse or pharmacist who treats you that you’re taking anticlotting medicines. If you have an accident and need emergency treatment, carrying a card or wearing a bracelet will alert medical staff to this.
  • Don't drink cranberry juice if you're taking warfarin because it can affect the way that warfarin works in your body. Eating large amounts of green vegetables, for example, broccoli, can have an effect on warfarin because they contain a lot of vitamin K, which affects how well it works. Alcohol and grapefruit juice have also been reported to interact with warfarin. Talk to your doctor for more information and before making any changes to your diet.
  • Look out for signs of abnormal bleeding such as excessive bruising, bleeding gums, blood in your urine, black faeces or nose bleeds. These symptoms may mean that your dose of warfarin is too high and it’s important that you seek urgent medical advice straight away.
  • Warfarin can damage an unborn baby so if you are or think you may be pregnant, it’s important that you get urgent advice from your GP about alternative anticlotting medicines.