Factor V Leiden: An Inherited Blood Clot Risk Explained — Symptoms, Causes, and Treatment
Updated: 2 days ago
Medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: September 2026
Source currency note: This guide reflects medical information reviewed in May 2026. Always confirm current guidance with your healthcare professional.
TL;DR
Factor V Leiden is an inherited change in one of the body's clotting proteins. It raises the risk of harmful blood clots, most often in the legs or lungs. The key point: most people with the gene never develop harmful clots, but those who do usually need blood-thinning (anticoagulant) medicine. A simple blood test confirms the diagnosis. The gene itself causes no symptoms — a blood clot in the leg (deep vein thrombosis) or a clot that travels to the lungs (pulmonary embolism) is often the first sign. Risk rises with a second gene copy, immobility (such as long flights), estrogen hormones, surgery, and certain blood types.
Quick Answer
What is factor V Leiden? An inherited change in a clotting protein that raises the risk of harmful blood clots, most often in the legs or lungs.
Does it cause symptoms? The gene change itself causes no symptoms. A blood clot may be the first sign of the condition.
Who is at risk? Anyone can have it. It is most common in people of white European descent, and a family history raises the risk.
How is it diagnosed? A blood test — either a blood-clotting study or a genetic test — ordered after one or more clots or a strong family history.
How is it treated? People with clots usually take blood thinners; people with the gene who have never had a clot don't need medicine. Daily habits like moving your legs on long trips lower risk further.
What is factor V Leiden?
Factor V Leiden (pronounced FAK-tur five LIDE-n) is a change in one of the clotting factors in the blood. A clotting factor is a protein that helps blood to clot.
This change can raise the risk of getting harmful blood clots, most often in the legs or lungs.
Here is the reassuring part: most people with factor V Leiden never get harmful clots. But people who do get them need to take blood thinners. Untreated, blood clots can lead to long-term health issues — and they can be life-threatening.
People of any sex can have factor V Leiden. Those who carry the changed gene may be more likely to get blood clots during pregnancy or when taking the hormone estrogen.
The good news is that treatment exists. Anticoagulant medicines, also called blood thinners, can lower the risk of getting more blood clots and having serious complications.
Factor V Leiden symptoms
Having the changed gene itself doesn't cause symptoms. For many people, a blood clot in the legs or lungs is the first sign of having the condition.
Some clots do no damage and go away on their own. Others can be life-threatening. The symptoms you notice depend on which part of the body the clot affects.
A clot in a deep vein (deep vein thrombosis)
A clot in a deep vein is called deep vein thrombosis, or DVT. It most often happens in the legs, and it may cause no symptoms at all.
When symptoms do appear, they can include:
Symptom | What you notice |
Pain | Aching or cramping in the affected leg |
Swelling | The leg, ankle, or foot looks puffy or larger |
Redness | Skin turns red (this may be harder to see on Black or brown skin) |
Warmth | The skin over the clot feels warmer than the surrounding area |
A clot that travels to the lungs (pulmonary embolism)
A pulmonary embolism happens when part of a clot in a deep vein breaks free and travels through the right side of the heart to the lung, where it blocks blood flow. This can be life-threatening.
Possible symptoms include:
Symptom | What you notice |
Breathing | Sudden shortness of breath |
Chest pain | Sharp pain that worsens when breathing in |
Cough | A cough that brings up bloody or blood-streaked mucus |
Heart rate | A rapid heartbeat |
Seek medical help right away if you have symptoms of either a deep vein thrombosis or a pulmonary embolism. Do not wait to see if symptoms improve on their own.
What causes factor V Leiden?
People who have factor V Leiden inherit the gene change from their parents. You can get one copy of the changed gene or, rarely, two copies — one from each parent.
The number of copies matters:
Gene copies | Inheritance | Clot risk |
One copy | Inherited from one parent (most cases) | Slightly raised risk |
Two copies | Inherited from both parents (rare) | Greatly raised risk |
Risk factors
A family history of factor V Leiden raises the risk of having the condition. The condition is most common in people who are white and whose families are from Eastern Europe.
People who inherit the changed gene from one parent have about a 5% chance of getting a blood clot by age 65. Several additional factors push that risk higher:
Risk factor | How it raises clot risk |
Two changed genes | Inheriting the gene from both parents greatly raises clot risk |
Not moving | Long periods of immobility — such as sitting during a long airplane flight — raise the risk of leg clots |
The hormone estrogen | Birth control pills, hormone replacement therapy, and pregnancy can raise clot risk |
Surgeries or injuries | Operations or injuries such as broken bones raise clot risk |
Certain blood types | People with blood types A, B, or AB have a higher risk than those with blood type O |
Complications
The main complications of factor V Leiden are the clots themselves. The condition can cause blood clots in the legs (deep vein thrombosis) and clots that travel to the lungs (pulmonary embolism). Both can be life-threatening.
How is factor V Leiden diagnosed?
A blood test can confirm the diagnosis. The test may be a blood-clotting study or a genetic test.
Your healthcare professional may order the test if you have had one or more blood clots, or if you have a strong family history of blood clots.
To confirm whether changed genes are the cause of your clots, you may be referred to one of two specialists:
Specialist | What they do |
Geneticist | A specialist in genetic conditions who can interpret genetic testing |
Hematologist | A specialist in blood conditions who manages clotting disorders |
Factor V Leiden treatment
Treatment depends on whether you have actually had a blood clot:
If you have had blood clots: healthcare professionals most often prescribe blood-thinning medicines to treat them and prevent new ones.
If you carry the gene but have never had a clot: you don't need this medicine.
Planning around surgery
If you have the gene change and need to have surgery, your healthcare professional might suggest extra steps to prevent blood clots:
Taking blood thinners for a short time around the operation
Using leg wraps that inflate and deflate to keep blood moving in your legs
Wearing compression stockings, which put pressure on your legs
Going for walks soon after surgery
Planning around pregnancy
If you are pregnant and have the factor V Leiden gene, your healthcare professional may suggest taking blood thinners during pregnancy and for a time after giving birth.
Lowering your risk at home
Several everyday habits can help lower your risk of blood clots:
Keep your legs moving. When your leg muscles tighten and relax, they help push blood through your body. On a long plane trip, raise your toes up and down and rotate your ankles every hour or so. Drink extra water to avoid losing fluids, and skip the alcohol. On car trips, take breaks and walk around.
Wear compression stockings. These usually come up to the knees and keep blood moving by pressing gently on your legs. Ask your healthcare professional whether they would help you.
Be careful with estrogen. Birth control pills and estrogen replacement therapy can raise clot risk. Talk with your healthcare professional before taking any medicine containing estrogen if you have factor V Leiden.
Preventing bleeding on blood thinners
If you take blood-thinning medicine, your blood takes longer to clot — so small cuts and bruises matter more. These steps help prevent injury and excessive bleeding:
Safety step | Why it helps |
Skip contact sports and injury-risk activities; choose gentle exercise like walking or swimming | Reduces the chance of injury that could cause serious bleeding |
Use a soft toothbrush and waxed floss | Protects gums from bleeding |
Use an electric razor instead of a blade | Prevents cuts while shaving |
Handle knives, scissors, and sharp tools carefully | Prevents accidental cuts during household tasks |
Preparing for your appointment
Your healthcare professional may send you to a geneticist or a hematologist to test whether changed genes are the cause of your blood clots and whether you have factor V Leiden.
Before you go, make a list of:
All your symptoms, and when they began
Your health history, including any history of blood clots and any family history of clots or factor V Leiden
All medicines, vitamins, and supplements you take, including dosages
Questions to ask your healthcare professional
Some useful questions for your main appointment include: What tests do I need? Do I need to see a specialist? Do I need treatment or clot-prevention medicine? What side effects can I expect? Do I need to limit my activity? Do my children need to be tested?
If genetic testing is suggested, consider asking the genetic specialist: How accurate is this test? What are its risks? What will a positive or negative result tell me? Could results affect my health insurance? What would an uncertain result mean? What are my treatment options? Should my children be tested? How long will results take?
Conclusion
Factor V Leiden is a common inherited gene change, and the most important fact to hold onto is this: most people who carry it never develop a harmful blood clot. But because the ones who do can face life-threatening clots in the legs or lungs, it deserves attention — especially after an unexplained clot or a strong family history.
Your next step: if you or a family member has had a blood clot without an obvious cause, ask your healthcare professional about a blood-clotting study or genetic test. If you already know you carry the gene, review your estrogen medicines, plan ahead before surgery or pregnancy, and learn the DVT and pulmonary embolism warning signs. And remember: sudden shortness of breath, chest pain when breathing, or a swollen, painful leg needs medical help right away.
Frequently Asked Questions
What is factor V Leiden?
Factor V Leiden is an inherited change in a blood-clotting protein that raises the risk of harmful blood clots, most often in the legs (deep vein thrombosis) or lungs (pulmonary embolism). It can affect people of any sex and any age.
Does factor V Leiden cause symptoms?
No — the gene change itself causes no symptoms. A blood clot in the legs or lungs is often the first sign of the condition.
Does everyone with factor V Leiden get blood clots?
No. Most people with factor V Leiden never get harmful clots. Those who do typically need blood-thinning medicine, which lowers the risk of further clots and serious complications.
How is factor V Leiden diagnosed?
With a blood test — either a blood-clotting study or a genetic test. It is usually ordered after one or more blood clots, or when there is a strong family history of clots.
Do I need medicine if I have the gene but no clots?
No. People who have the gene change but have never had blood clots do not need blood-thinning medicine. The focus is on lowering risk through movement, compression stockings, and careful use of estrogen.
Should my children be tested?
Because factor V Leiden is inherited, children of a carrier can carry the gene too. Ask your doctor or a genetic specialist whether testing makes sense for your family — this is one of the key questions to bring to your appointment.
Related guides on Rinnit
If hormones and blood clot risk are on your mind, see our coverage of HRT and blood clots: menopause hormone pills vs. patches in a major 2026 study. And for a closer look at clots forming in the veins, see our guide to thrombophlebitis: symptoms, causes & treatment.
References
This article was prepared for general information only and is not a substitute for professional medical advice.
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