Reviewed by The Wellness Team. Last reviewed October 2026.
Long flights and blood clots are linked, but the real risk is smaller than most people fear and very manageable once you know whether you are in the group that needs to take it seriously. For a healthy traveller, the chance of a symptomatic clot after a long-haul flight is roughly 1 in several thousand journeys; for someone with risk factors it is much higher and worth planning around. Flying long-haul soon? Message us on WhatsApp and a doctor can assess your personal risk before you go.
Can flying give me a blood clot?
Yes, but the absolute risk for most people is low. Sitting immobile for more than about four hours slows blood flow in the deep veins of the legs, which is what allows a deep vein thrombosis (DVT) to form. The WHO Research Into Global Hazards of Travel project found the risk of a symptomatic clot roughly doubles to triples after journeys over four hours, rising with each additional hour, and is highest in the first couple of weeks after travel. In absolute terms that is still about one event per 4,500 to 6,000 long-haul flights in healthy travellers.
The clots that matter are the ones that travel to the lungs (a pulmonary embolism), which is why prevention focuses on people whose baseline risk is already raised.
Who has a higher risk when flying?
As of October 2026, NICE guidance and the Aerospace Medical Association identify the travellers who should plan ahead rather than just hope for the best:
- A previous DVT or pulmonary embolism
- Recent surgery (within about four to six weeks, especially hip, knee or pelvic surgery) or recent major injury
- Active cancer or recent cancer treatment
- Pregnancy and the first six weeks after birth
- Oestrogen-containing contraception or hormone therapy (this raises clotting tendency; it is a factor to discuss, not a reason to panic)
- Thrombophilia (an inherited clotting tendency), obesity, or significant limited mobility
If none of these applies, sensible in-flight habits are all you need. If several apply, or you have had a clot before, book a pre-travel assessment: high-risk travellers may be advised to use properly fitted compression stockings, and in specific cases a doctor may recommend preventive medication for the journey. That is an individual decision that depends on your history, so WhatsApp us before you fly rather than guessing.
What actually prevents DVT on a long flight?
The measures with evidence behind them are simple:
- Move every hour or two: walk the aisle when the seatbelt sign is off, and do seated calf raises and ankle circles (ten of each, each hour) while seated
- Stay hydrated with water, and go easy on alcohol, which dehydrates you and makes you sleep in one position
- Keep space under the seat in front free so you can stretch your legs; avoid crossing your legs for long stretches
- Below-knee graduated compression stockings if you have risk factors: they meaningfully reduce symptomless clots on long flights in studies, and they need to fit properly to work
What does not help: aspirin. It is not recommended for travel clot prevention, because it barely affects venous clots and carries bleeding risk.
What are the warning signs after a flight?
A DVT usually shows up days to two weeks after travel as one-sided calf or thigh swelling, pain or tenderness (often worse when standing), warmth and sometimes redness, always in one leg rather than both. Seek same-day medical care for those symptoms.
A pulmonary embolism is an emergency: sudden breathlessness, sharp chest pain that is worse on breathing in, a rapid heartbeat, coughing blood, or feeling faint. Call 999 or go to an emergency department immediately and tell them you recently flew.
Can an online GP help before a long trip?
Yes. A pre-travel consultation maps your risk factors, advises on stockings and fit, reviews anything in your history that changes the advice (recent surgery, cancer, pregnancy, clotting disorders), and arranges any preventive plan that is appropriate for you. If you need documentation for travel, we also handle travel health and fitness-to-fly certificates.
As of October 2026, a phone consultation with The Online GP costs £59, a video consultation costs £150, and an in-person appointment at 10 Portman Square, Marylebone costs £220. Message us on WhatsApp and you can usually speak to a doctor the same day.
Frequently asked questions
Does flying in economy cause DVT (the so-called economy class syndrome)?
The seat class is not the issue; immobility is. Business class passengers who stay seated for eight hours face the same mechanism. Window seats carry higher risk than aisle seats simply because people in them move less.
How long after a flight am I at risk?
The elevated risk is highest in the first two weeks after travel and returns to normal over the following weeks. Symptoms of a travel-related clot almost always appear within a month.
Should I wear compression stockings?
If you have any of the risk factors listed above, below-knee graduated compression stockings are a reasonable precaution with decent evidence behind them. Fit matters more than brand; a doctor or trained fitter can confirm the right size.
I had a DVT years ago. Can I fly?
Usually yes, with precautions, but this is exactly the situation that benefits from a pre-travel assessment rather than generic advice. Message us and we will review your history.
Does drinking water really make a difference?
Hydration alone does not prevent clots, but dehydration concentrates the blood slightly and discourages walking to the lavatory. It is one easy part of the overall picture, alongside movement.
Sources: NICE Clinical Knowledge Summaries, venous thromboembolism prevention for travellers; Aerospace Medical Association medical guidelines for air travel; WHO Research Into Global Hazards of Travel (WRIGHT) project.
Reviewed by The Wellness Team.