How to prevent blood clots—plus spotting the symptoms

Expert advice on why they happen, who's at risk, and how to prevent blood clots

how to prevent blood clots, human blood cells
(Image credit: Getty Images / Sebastian Kaulitzki/Science Photo Library)

Knowing how to prevent blood clots could be a lifesaver. Blood clots are essential to stop us bleeding to death from minor cuts, but they can also form where vessels become blocked. This can happen as a result of weight gain, an unhealthy diet, inactivity, smoking, or even medication–causing major problems.

Shockingly, more people die from blood clots than breast cancer and road traffic accidents combined. What's more, it’s also the number one cause of preventable deaths in hospitals, so knowing how to prevent blood clots is essential.

We reveal everything you need to know—including how blood clots form, who's at risk, and how to prevent them.

How to prevent blood clots, lab test equipment

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1. Blood clots can occur inside and outside the body

Blood clots form when blood turns from a liquid into a semisolid mass. 

"Clotting is a natural process that 'plugs' a hole and stops you losing too much blood in the event of an injury—on the outside this is known as a scab," explains Professor Mark Whiteley, a leading consultant venous surgeon. 

However, when platelets, plaque or cholesterol build up inside a vessel this creates an obstruction or clot, and can be life-threatening. 

2. There are three main causes of blood clots

Blood doesn't normally clot inside our vessels, so something has to change in order for clots to appear. 

"About 100 years ago a doctor called Virchow described three things that can change to cause blood to clot," explains Professor Whiteley. These include changes in your blood composition (which include concentration levels of plasma, red blood cells, white blood cells and platelets); changes in your blood flow (if it becomes sluggish or slow) and changes in the vessel wall (if it's damaged by lifestyle factors). 

"This is now known as "Virchow's triad"—if any one of these changes occurs, then you can get a blood clot," he adds.

3. Stopping smoking is an easy way to prevent blood clots

Knowing how to prevent blood clots is key to staying healthy and avoiding heart attacks or strokes. "Artery clots are usually caused by things that are bad for your circulation," says Professor Whiteley. 

Smoking is particularly bad, as it thickens the blood and damages vessel walls. "The walls of our blood vessels are lined with a special sort of cell which prevents normal blood from clotting on it, but smoking changes the surface of these blood platelets, making it much easier for them to stick together and form clots," explains Professor Whiteley. 

Artery walls can also be damaged by high blood pressure, high levels of fat in the blood from your diet, and diabetes.

4. Varicose veins can result in blood clots

A varicose vein occurs when veins bulge or become twisted. This happens when a valve in the vein fails and blood begins to collect in the vessel, rather than continuing towards the heart. 

Blood flow through them then becomes sluggish or slow, which can cause superficial blood clots, known as thrombophlebitis, phlebitis or superficial venous thrombosis.

Constricted and narrowed artery

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5. Clots in different parts of your body cause different symptoms

"Clots in different parts of your body can be caused by different things," explains Dr Emeka Okorocha, and can impact your body in different ways.

  • ARTERIES
    A blood clot in an artery can be serious, especially if a blockage occurs in one leading to the heart, which can cause a heart attack or in the brain which can cause a stroke. They usually cause sudden and severe pain, and in the case of stroke can be accompanied by limb weakness and difficulty speaking and seeing. But don’t worry there are simple lifestyle changes you can make to keep you healthy and significantly reduce your risk.
  • DEEP VEIN OF LEG
    A blood clot in a deep vein of the leg (deep-vein thrombosis or DVT) causes a swollen leg and pain. There’s a risk some of the clot could break off and travel to your lungs— causing a pulmonary embolism—but there are simple ways to reduce your risk.
  • SUPERFICIAL VEINS OF THE LEG
    A blood clot in the superficial veins of the leg causes superficial venous thrombosis (often called phlebitis). This is usually just painful, but if the clot extends near the deep vein it could travel to your lungs too, so it’s good to get these checked out.

6. All blood clots need urgent medical attention

Professor Whiteley worries that blood clots in "superficial" veins are not treated seriously enough. 

"Everyone with "phlebitis" in the legs should be sent to a venous specialist for an ultrasound scan [to monitor blood flow]," he says. "Anyone who has a clot close to the deep veins must have anticoagulation treatment (drugs to thin the blood) to stop the clot from travelling to their lungs."

Woman pouring a glass of water

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7. Dehydration can cause blood clots

Knowing how to prevent blood clots could be as simple as drinking a glass of water. "When people are dehydrated the composition of their blood changes and becomes much more concentrated," says Professor Whiteley. 

It’s vital to drink plenty of water to thin the blood and keep it flowing. Reduce caffeine and alcohol intake, too, which can make you more dehydrated. Loma Linda University researchers found that people who drank five or more 250ml glasses of water daily cut their stroke risk by 53%. 

8. Staying mobile can prevent blood clots

A staggering 60% of blood clots happen during or after a hospital stay, due to inactivity. How to prevent blood clots when you're sedentary in hospital or even on a long haul flight can be tricky, but there are things you can do.

"In hospital, there are a number of blood tests to determine somebody’s risk of stroke, heart attack or pulmonary embolism," explains Dr Okorocha. "You may be given blood thinning tablets and surgical stockings to reduce the risk." 

On a plane, know how to prevent blood clots. Keep your legs moving, even when sitting. Try extending your legs straight out and flexing your ankles, or pull each knee in towards your chest.

9. Blood clot symptoms can be tricky to spot

Spotting blood clot symptoms could be key in helping you know how to prevent blood clots becoming fatal. 

"Symptoms can often be deceptively mild and can be attributed to a range of conditions," says Dr Okorocha. But if you notice any of these symptoms, and think you may have a blood clot, seek urgent medical help. 

  • Sudden shortness of breath and chest pain, particularly when you breathe in
  • Coughing up blood
  • Painful, swollen leg
  • A tender calf, particularly when standing
  • A red patch on your leg that feels warm to touch
  • Sudden and severe headache
  • Dizziness
  • Numbness or weakness, particularly on one side of your body

Women running on a trail is a way how to prevent blood clots

(Image credit: Getty Images / Erik Isakson)

10. Changing your lifestyle is an easy way to prevent blood clots

Changes you make in your daily lives could be key in how to prevent blood clots developing. "Lifestyle is a major factor in preventing or causing blood clots," says Dr Okorocha. 

"You cannot change some risk factors, like age or family history, but everyday behaviours or habits are within your control and can drastically help to reduce the risks." You can help to prevent blood clots by:

  • Maintaining a healthy weight (BMI 19-25)
  • Having healthy blood pressure and cholesterol levels
  • Reducing salt and sugar intake
  • Staying hydrated
  • Exercising – the NHS recommends around 150 minutes per week
  • Keeping moving and wearing compression socks during a long flight
  • Elevating your feet and legs while sitting or lying down to stop your blood from pooling 
  • Stopping smoking
  • Speaking to your doctor if you have a family history of blood clots and are taking oral contraceptive. "They may be able to prescribe medication," says Dr Okorocha.