Blood clots, thrombosis, and the contraceptive pill

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If you have a cut, and don’t have a medical condition, you notice that you bleed to start with and then quite quickly this bleeding stops and a scab forms. This sticky collection of some of the cells in your blood is a blood clot (or thrombus), and it is entirely harmless. In fact, it is an essential part of the healing process.

However, blood clots forming when they aren’t supposed to can cause problems, and you may have read articles about women who are taking the combined oral contraceptive pill (COCP) developing blood clots.

A venous thrombosis is a clot in the vein, usually diagnosed in the leg, where it can cause pain, swelling and discolouration. These clots can become dislodged and travel through the bloodstream to a lung, resulting in pulmonary embolism. This is a blockage of the vessels that supply the lung, and symptoms include chest pain, breathlessness, and sometimes coughing up blood. These events are collectively termed venous thromboembolism (VTE).1

Very occasionally, a clot can occur in the large veins that carry blood away from the brain. This is known as venous sinus thrombosis, and will usually cause a headache.2

Contextualising the risk

While that sounds very dramatic, and while we should take steps to prevent it from happening, the actual risk of dying after developing a clot is about 1%.3 Of every one million women using the pill, between three and 10 women are estimated to die each year as a result of VTE events attributable to use of the pill.4 To put this into context, 12 women of reproductive age die in a road traffic accident every year.

In any case, without doing anything at all, your risk of developing VTE as a woman of reproductive age is 4-5/10,000 women-years. So, of every 10,000 women of reproductive age, about five of them will develop a blood clot in a deep vein in any one year, even if none of them are taking the pill.

If you had 10,000 pregnant women, 30 of them would develop blood clots in their deep veins, usually of the leg, in one year, so the risk of developing a blood clot while pregnant is 30/10,000. In the weeks following the delivery of their babies, three hundred of them would be at risk of developing clots.

As mentioned, taking the COCP does indeed carry an increased risk of developing clots, a risk of 5-9/10,000. So, it’s a higher risk than not taking it, much less than the chances of developing a blood clot if you were to get pregnant instead.

Why does the pill slightly increase risk of blood clots?

The combined pill consists of two types of hormones, an oestrogen and a progesterone. Blood clot risk is primarily increased by the use of oestrogens, and some specific types of ‘newer generation’ progesterone compounds.

Different pill brands contain different varieties of oestrogens and progesterones, at different doses. This is so that you and your doctor can find the best fit for you depending on why you are taking the pill. The hormones do not make clots in your blood, but they do increase the amount of clotting factors that you have in your blood, thus increasing your risk of developing a blood clot. Clotting itself is caused by a bunch of different hormones and enzymes that are set up to work in a domino effect. The COCP also decreases the anti-clotting factors in your blood.

With different types of oestrogen, the more hormone that is in the pill, the greater the risk it poses. With progesterones only certain types (or generations) increase risk.

Why the pill is relatively safe

The contraceptive pill is a medication that allows women to plan pregnancy, regulate irregular and heavy bleeding, and manage side effects related to hormone imbalances in the body.

The pill has been around in some form since the 1960s, and the amount of type of hormone each pill contains has been refined throughout the years. It is widely available, and an estimated 3.2 million women were on some form of the pill in 2000.5

As with all medical interventions, it is not risk free. However, it is important to put this risk into context in order to make an informed decision. In the absence of any other factors, your absolute risk of developing a blood clot still remains low, even when you’re taking the COCP. That’s not to minimise the risk however, but it does help to keep the risk in context when making your decision.

Who shouldn’t take the combined oral contraceptive pill?

So, is the combined oral contraceptive pill something you should still consider taking? Guidance from the FSRH states that the majority of women can use combined hormonal methods of contraception without harm.

However, they also state: “There are some medical conditions and lifestyle factors that are associated with either theoretical or proven health risks if a combined hormonal method is used. Women should be empowered to make informed decisions about choosing and using the COCP.”6

There are certain instances when you definitely shouldn’t take the COCP. These include if you have had a deep vein thrombosis in the past or have a medical condition that predisposes you to blood clots, as this poses an unacceptable health risk to you.

You should also not take the COCP if you suffer from high blood pressure (BP 165/95mmHg) or migraine with aura, or if you are a smoker, or have a body mass index of more than 35kg/m2.

Your doctor will advise you to stop taking the COCP temporarily if you are hiking at an altitude of greater than 4500 meters. (Ben Nevis is only 1400 meters, Everest is almost 9000 meters. You know if you are a high-altitude hiker!) In all of these situations your doctor will offer you alternative forms of contraception.

As a side note, if you are on the COCP and taking a flight lasting more than three hours, it is important that you walk around and stay well hydrated on your flight to reduce your risk of blood clots.

Are there any signs or symptoms that may be indicative of a blood clot? The tell tale signs begin around the area of the clot, usually (but not always) in the calf of one leg. You may notice that one leg is obviously more swollen than another, and that a specific area is painful or discoloured.7 More serious is if you have seen this and then developed problems with your breathing, or have begun feeling that you can’t quite catch your breath. If you think you may have a blood clot it is important for you to seek medical attention immediately.

Always speak to a doctor before beginning a new form of contraception so you can ensure you are taking one that is safe and suitable for you.

Last updated December 2017
Next update due December 2019

Disclaimer: This content is designed to provide general information and is not a substitute for medical advice. Medical opinion, practice and routine may vary from country to country and may change from time to time. The author is not liable for the results of misuse or inappropriate application of the information. If you are an individual who chooses to access this information, you should not rely on the information as professional medical advice or use it to replace any relationship with your doctor or other qualified healthcare or social care professional. For medical concerns, including decisions about medications and other treatments, individuals should always consult their doctor or, in serious cases, seek immediate assistance from emergency personnel. Individuals with any type of medical condition are specifically cautioned to seek professional medical advice before beginning any sort of health treatment. Individuals with specific queries or concerns should seek appropriately qualified medical advice. Clinicians must use their own judgement when interpreting this information and deciding how best to apply it to the treatment of patients.


  1. FSRH Clinical Effectiveness Unit, ‘Combined hormonal contraception and venous thromboembolism’, Statement from Clinical Effectiveness Unit, Faculty of Sexual and Reproductive Healthcare, 2016, [website],, (accessed 4 December 2017).
  2. Ibid.
  3. K. Hedenmalm et al., ‘Fatal venous thromboembolism associated with different combined oral contraceptives: a study of incidences and potential biases in spontaneous reporting’, Drug Saf., Vol. 28, No. 10, 2005, pp. 907-916.
  4. FSRH Clinical Effectiveness Unit, ‘Combined hormonal contraception and venous thromboembolism’, 2016.
  5. Family Planning Association, ‘Contraception: patterns of use factsheet’, [website], 2007, (accessed 4 December 2017).
  6. FSRH Clinical Effectiveness Unit, ‘Combined Hormonal Contraception’, Clinical Guidance, Faculty of Sexual and Reproductive Healthcare, [website], 2011,, (accessed 4 December 2017).
  7. FSRH Clinical Effectiveness Unit, ‘Combined hormonal contraception and venous thromboembolism’, 2016.

Dr. Natasha Abdul Aziz

Natasha is senior clinical fellow in Obstetrics & Gynaecology with the Chelsea & Westminster NHS foundation trust. She was previously a clinical research fellow at the University of Oxford and national women’s health lead for the Muslim Doctors Association of the UK. Her special interests include the use of digital healthcare solutions to help vulnerable populations of women, volunteer work with refugees, and chasing that last sliver of sun on her balcony after work.

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