Not just COVID-19 vaccines: 5 things that cause blood clots

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Reports of COVID-19 vaccinations causing blood clots has caused alarm in the general population. However due to the rarity of incidences from vaccination, myGP explores the most common causes of blood clots and how we can prevent them.

Over 11 million doses of COVID-19 vaccines have been given across the UK, with blood clotting observed in a small sample of patients. However, despite the reported risk the European Union has approved the safety of the AstraZeneca vaccine after pausing its rollout due to safety concerns.

As we know, any medication that goes into circulation is subject to rigorous testing – no corners are cut, no matter how urgent the need. In the 1950s, the thalidomide scandal resulted in tighter testing on drugs when tens of thousands of babies were born with birth defects when pregnant women were prescribed it, so it is understandable that concerns around vaccinations and drug prescriptions must be investigated.

Interest in blood clots has surged because of recent news. myGP looked at Google Trend data to find out what users were searching for – assigning a score from 0 to 100 based on interest, and 100 being the highest. According to the data, on 13 March 2021, interest scores for “blood clot symptoms” in the UK was 11, which is low. On the 15 March 2021, it had risen to 100. Similarly, the interest score for "AstraZeneca blood clot" in the UK was on 0 on 11 March and skyrocketed up to 100 on the 14 March.

After reports of blood clots following the AstraZeneca vaccine for COVID-19, there was understandably a need for these to be investigated. However, it is important to acknowledge that the vaccine has since been approved again as "safe and effective" and concerns of blood clots were debunked.

If you’re concerned about the risk of blood clots, we’re here help give you accurate information about things that can contribute to blood clots, and how you can prevent them.


Smoking can contribute to a vast majority of health problems, so it isn’t surprising that smoking cigarettes is on the list. As well as causing significant damage to organs in our body, smoking is the leading preventable cause of disease and death. Smoking significantly increases your risk of blood clots and will contribute to platelets – the small cell fragments in our blood that clot to prevent bleeding – sticking together. Smoking also damages blood vessel lining, which can cause clots to form.

Atherosclerosis can also arise from smoking, where plaque in the blood builds and sticks to the artery walls. These plaques of blood make your arteries smaller which reduces blood flow and can lead to blood clots. Smoking also has links with a heart attack, blockages, stroke and coronary heart disease.

Quitting smoking can be difficult – after all, it is an addiction. However, people shouldn't feel like they are a “lost cause” and that the damage has already been done. Studies have shown there are health benefits even after eight to 12 hours of quitting smoking. Your blood carbon monoxide levels drop. Two to three weeks after quitting, your risk of heart attack drops. One year after quitting, your risk of heart disease will be cut in half. Ten years after quitting, your risk of lung cancer falls to the same level of someone who never smoked.


Oestrogen-based contraceptives can contribute to your risk of a blood clot. Not only the pill, but patches, injections, or vaginal rings that deliver estrogen to the blood stream can all cause an increased risk. Because of hormone changes, this can cause blood cells to form clots.

The risk of blood clots when on oestrogen-based contraceptives is higher in:

  • Those with a family history of blood clots.
  • Those who have had surgery.
  • Those who are obese.
  • Those who go on prolonged travel.

Sometimes, the contraception may have fewer side effects on a specific type, we can access it easily and order prescriptions online, or generally prefer a certain method. To reduce your risk of blood clots while on oestrogen-based contraception, you should:

  • Maintain a healthy weight with diet and regular exercise.
  • Drink enough water, particularly when travelling.
  • Wear compression socks – get advice from your GP on this.
  • Be aware of the symptoms – these may include chest pain, shortness of breath, upper body discomfort including in the arms, back, jaw, or neck, speech changes, paralysis, trouble speaking, and redness, pain, warmth, or swelling in the lower leg.


Did you know that if you're pregnant or have recently given birth, you are at higher risk of blood clots? Pregnant women are five times more likely to have a blood clot than women who aren't pregnant.

Due to changes a woman's body will go through, including less blood flow getting to the legs of pregnant women due to the pressure of a growing baby on the pelvis and limited mobility, this can increase the risk of blood clots.

You should speak to your doctor about your risk and if you have any and be aware of the symptoms.

Prolonged bed rest

Blood clots can arise due to prolonged bed rest. It is important to keep your blood flowing when you're immobile for a significant period of time. For example, if you're ill or are recovering from surgery, you need to keep your limbs active to encourage blood flow.

Compression stockings can also aid with this, designed to apply pressure to your legs to maintain blood flow.


Lastly, and most importantly – COVID-19. COVID-19 can cause blood clots, not the vaccination against it. Blood clots are seen in people who have been hospitalised with the virus. According to Healthline, 31% of these patients had blood clot related complications – it isn’t just the elderly at risk, but young people too.

To reduce risk, it's important to maintain a healthy weight, regularly exercise, drink water, and avoid smoking.

About the author

myGP is the UK's largest independent GP booking and healthcare management app that connects patients with primary care. The app can be used to book GP appointments, order repeat prescriptions and set up medication reminders on smartphones.