This leaflet advises on how to reduce the risk of a deep vein thrombosis occurring during a long journey. Another leaflet discusses deep vein thrombosis in more detail. If you need more detailed advice on this issue, you should consult a private travel doctor, or your consultant if it is about a particular medical condition that you have. A GP can provide you with a printout of your medical history, but will generally not be able to advise on the use of blood thinners when travelling, or provide a letter to say that you are 'fit to fly' as they are not trained or insured to do so.
What is a deep vein thrombosis?
A deep vein thrombosis (DVT) is a blood clot that forms in a deep vein, usually in the leg. The deep veins in the leg are blood vessels which go through the calf and thigh muscles, and are not those which you can see just below the skin.
What is travel-related deep vein thrombosis?
Long journeys (more than four hours) by plane, train, bus, car, etc, are thought to cause a slightly increased risk of DVT. This is probably due to sitting immobile and cramped for long periods. Blood flows more slowly and collects in the legs when they are hanging down. Blood flowing slowly is more likely to make a clot. Symptoms of a DVT include pain, redness or swelling in one leg. It is diagnosed by an ultrasound scan - if you have symptoms, you might also have a blood test to see if your risk is high or low.
The risk of DVT from travel is small. Research studies suggest that there is about one DVT for every 4,656 flights that last for four hours or more. The longer the flight, the more likely you are to develop a DVT. It has to be stressed that the vast majority of travellers have no problems. Other risk factors are involved, so for most people the chance of developing a DVT just from a long journey is very small.
Is a deep vein thrombosis serious?
It can be. When a blood clot forms in a leg vein, it usually remains stuck to the vein wall. There are two main possible complications:
- A blood clot in the lung (pulmonary embolus) occurs in a small number of people who have a DVT. An embolus is a part of a blood clot which breaks off and travels in the bloodstream. An embolus that comes from a clot in a leg vein may be carried up in the bloodstream and can become stuck in one of the blood vessels of the lung. This is serious and can sometimes be fatal. It can present with chest pain or shortness of breath.
- Long-term discomfort and swelling of the calf occur in some cases following a DVT (post-thrombotic syndrome).
See the separate leaflet called Deep Vein Thrombosis for more details.
Who is at risk of having a deep vein thrombosis when they travel?
Anyone may develop a DVT following a long journey but the following increase the risk:
- Having had an operation in the previous two months.
- Having had a DVT or pulmonary embolism (PE) before. (If you have had a DVT or PE before and are still taking anticoagulant medication, you are not at a higher risk.)
- Having a close relative who has had a DVT or PE.
- Being on combined contraceptive hormone pills, patches or rings and oral hormone replacement therapy (HRT).
- Some blood clotting disorders (for example, polycythaemia or thrombophilia).
- Having a broken leg which is in plaster.
- Recent severe illness such as pneumonia or heart failure or a heart attack.
- Having combined obstructive pulmonary disease (COPD).
Remember, even if you have one of these risk factors, the chance of developing a DVT after a journey is still small. However, it makes sense to try to reduce even this small risk if possible.
How to reduce the risk of DVT when travelling
Whilst travelling on a long journey:
- Exercise your calf and foot muscles regularly:
- Every half hour or so, bend and straighten your legs, feet and toes when you are seated.
- Press the balls of your feet down hard against the floor or footrest every so often. This helps to increase the blood flow in your legs.
- Take a walk up and down the aisle every hour or so, when it is safe to do so.
- Make sure you have as much space as possible in front of you for your legs to move. So avoid having bags under the seat in front of you and recline your seat where possible.
- Take all opportunities to get up to stretch your legs, when there are stops in your journey.
- Drink normal amounts of fluid to avoid a lack of fluid in the body (dehydration).
- Do not drink too much alcohol. (Alcohol can cause dehydration and immobility.)
- Do not take sleeping tablets, which cause immobility.
Elastic compression stockings
There is some evidence to suggest that compression stockings can help to prevent travel-related DVT in people who have a high to moderate risk. A 2021 review quantified this as a reduction in the risk of DVT from about 10 - 30 to 1-3 per 1000 passengers, so a reduction of 90%. You can buy the stockings from pharmacies. Ask the pharmacist for advice about the correct sort. Air travel guidelines recommend these for passengers who are at moderate risk, for example if you are aged over 60, have extensive varicose veins, have had recent minor surgery or are pregnant.
They need to be 'graduated compression' stockings, with the correct amount of compression (class 1). The slight pressure from the stocking helps to prevent blood 'pooling' in the calf. Stockings do not replace the need for regular exercises. They are in addition to exercises.
If you have a plaster cast on your leg, this would usually be split for the journey. Consult your fracture clinic for advice. A GP will not be able to give advice on this, as it is a specialist area - allow plenty of time before flying to get an opinion from the fracture clinic.
Some people at particularly high risk of DVT may be advised to have a heparin injection before a long-haul flight. Heparin is an anticoagulant (blood thinner), meaning it 'thins the blood' and makes it less likely to clot. See your specialist before the journey to discuss this option if you:
- Have thrombophilia.
- Have cancer which is being treated.
- Have had major surgery in the previous four weeks. Specifically if you have had an operation under a general anaesthetic lasting more than 30 minutes. If you have had a hip or knee replacement within the previous three months, you should avoid long-haul flights altogether.
Your GP will not be able to advise on the use of anticoagulants for flying as it is a specialist issue, so allow plenty of time to get an opinion from your specialist.
Note: aspirin is not used to prevent DVT. (Aspirin is widely used to help prevent the risk of blood clots in arteries, which can cause strokes and heart attacks. However, aspirin does not seem to be very effective at preventing clots in veins.)
Further research may clarify the value of compression stockings and heparin, or find other ways of preventing DVT.
After the journey
Have a little walk straight after the journey to 'get the circulation going'. The vast majority of travellers have no problems. However, if you develop a swollen painful calf or breathing difficulties shortly after a long journey then see a doctor urgently. Note: slight painless puffiness of feet and ankles is common after a long journey and is not due to a DVT.
With thanks to Dr. Clare Hunter, BSc MBBS DAvMed MRAeS MRCGP(2009) DOccMed for her input into this leaflet.
Further reading and references
Prevention and management of venous thromboembolism; Scottish Intercollegiate Guidelines Network - SIGN (December 2010, updated October 2014)
DVT prevention for travellers; NICE CKS, August 2018 (UK access)
Watson HG, Baglin TP; Guidelines on travel-related venous thrombosis. Br J Haematol. 2011 Jan152(1):31-4. doi: 10.1111/j.1365-2141.2010.08408.x. Epub 2010 Nov 18.
Izadi M, Alemzadeh-Ansari MJ, Kazemisaleh D, et al; Do pregnant women have a higher risk for venous thromboembolism following air travel? Adv Biomed Res. 2015 Feb 234:60. doi: 10.4103/2277-9175.151879. eCollection 2015.
Air Travel; British Thoracic Society, 2022
Clarke MJ, Broderick C, Hopewell S, et al; Compression stockings for preventing deep vein thrombosis in airline passengers. Cochrane Database Syst Rev. 2021 Apr 204(4):CD004002. doi: 10.1002/14651858.CD004002.pub4.
Travel-Related Venous Thrombosis; British Society for Haematology 2010 (addendum added 2023)