Thrombosis Australia

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The "pill" can increase your chances of thrombosis

The combined oral contraceptive pill is commonly prescribed to women during their childbearing years. The “pill” contains both a synthetic oestrogen and progesterone. Oestrogen can increase the levels of “clotting factors” within your blood. These factors make it easier for your blood to clot and increases your risk of venous thromboembolism (VTE) - which includes deep vein thrombosis (DVT) and pulmonary embolism (PE). The highest risk is during the first year of use.

The pill can increase your risk of developing thrombosis by 2- to 3-fold compared to those not taking it(1)

However, it is important to remember that the absolute risk is still quite small. For this reason, the advantages of taking the pill can outweigh the added risk of VTE. If you are predisposed to VTE your doctor will help you to decide what the best option for you is. For instance, they might recommend a different contraceptive pill with less or no oestrogen component.  

Who is more at-risk of thrombosis?

There are many risk factors for thrombosis which are important to be aware of to avoid and prevent thrombosis.

Being unable to move around:

  • Hospitalisation
  • Long-term bed rest
  • Paralysis
  • Long-term sitting - such as travel

Surgery and Injury:

  • Major injury - especially of the legs and lower body
  • Major surgery - especially of hips and legs
  • Catheterisation

Other medical conditions:

  • Cancer and cancer treatment
  • Heart failure or heart disease
  • Thrombophilia (clotting disorder)
  • Obesity/overweight


  • Birth control which contains oestrogen
  • Hormone replacement therapy
  • Pregnancy and the 6 weeks following birth


  • Smoking
  • Family history of thrombosis
  • Age over 50
  • Varicose veins

If any of these risk factors apply to you it is important to discuss this with your doctor.

How can you reduce the risk of thrombosis?

It is important to talk to your doctor and understand the risk factors for venous thromboembolism (VTE) before using the combined oral contraceptive pill. Occasionally people might be advised to take extra precautions – such as wearing compression stockings during travel or taking medication to help thin your blood for a short period of time.                                                                              

Contraception and thrombosis FAQs:

How does the contraceptive pill increase the risk of thrombosis?

The increased risk of thrombosis when taking the combined oral contraceptive is generally attributed to oestrogen. The pill contains both oestrogen and progesterone and is designed to imitate pregnancy to prevent it. Oestrogen increases the levels of clotting factors in the blood which increases the likelihood of developing a clot.

What is the increased risk of developing thrombosis while on the pill?

The increased risk while on the pill is 2- to 3-fold. This is still only around 1 in 3000 women who develop a clot while taking birth control pills. However, those with a history of thrombosis, or an inherited thrombophilia, have an increased risk.

Can I reduce my risk of thrombosis and still take the birth control pill?

Yes. Depending on your risk, there are alternative choices for you that can reduce your risk. If you have a thrombophilia or history of thrombosis then your doctor may recommend that you take blood-thinners to mediate your risk.

Your doctor may also recommend taking progesterone-only contraceptives which do not appear to increase the risk of thrombosis.

Learn more about thrombosis.

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  1. Stegeman, B., de Bastos, M., Rosendaal, F., Vlieg, A., Helmerhorst, F., Stijnen, T., and Dekkers, O. Different combined oral contraceptives and the risk of venous thrombosis: systematic review and network meta-analysis. BMJ 2013;347:f5298.
  2. Bateson, D., Butcher, B., Donovan, C., Farrell, L., Kovacs, G., Mezzini, T., Raynes-Greenow, C., Pecoraro, G., Read, C., and Baber, R. Risk of venous thromboembolism in women taking the combined oral contraceptive: A systematic review and meta-analysis. The Royal College of General Practitioners 2016; 45(1-2).
  3. Dragoman, M., Tepper, N., Fu, R., Curtis, K., Chou, R., and Gaffield, M. A systematic review and meta-analysis of venous thrombosis risk among users of combined oral contraception. Int J Gynaecol Obstet. 2018; 141(3): 287-294.