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How the pill affects blood clotting
The combined oral contraceptive pill contains oestrogen and progestogen. Oestrogen is the main hormone linked to a higher risk of blood clots. It can make the blood slightly more likely to clot than it would otherwise.
This does not mean the pill causes a clot in most people. For most users, the overall risk remains low. But the risk is higher than for people who do not take hormonal contraception.
Why the risk goes up
Blood clots form when the body’s clotting system becomes more active than usual. Oestrogen can increase some clotting factors and reduce natural proteins that help prevent clots. This shifts the balance towards clot formation.
Because of this, a clot may be more likely to develop in a vein, especially in the legs. If a clot travels to the lungs, it can cause a pulmonary embolism, which is a medical emergency.
How big is the risk?
The risk of a blood clot is still small for most healthy women taking the combined pill. However, it is higher than in women of the same age who are not using it. The risk is also lower than the risk during pregnancy and after childbirth.
Different pills carry slightly different levels of risk. Some types of combined pills are linked with a greater chance of clots than others. Your GP, nurse or pharmacist can help you understand which option may be suitable for you.
Who is at higher risk?
Some people have a greater chance of developing a clot while taking the pill. This includes women who smoke, are over 35, have obesity, or have a personal or family history of blood clots. Certain medical conditions can also raise the risk.
Long periods of immobility, such as after surgery or during a long journey, can add to the risk as well. If you have one or more risk factors, a clinician may suggest a different contraceptive method. Progestogen-only options may be safer for some people.
Warning signs to look out for
It is important to know the symptoms of a possible clot. Signs of a deep vein thrombosis include swelling, pain, warmth or redness in one leg. A clot in the lung may cause sudden breathlessness, chest pain or coughing up blood.
If you think you may have a clot, call 999 or go to A&E straight away. Do not wait to see if symptoms improve. If you are worried about your pill, speak to your GP, practice nurse or a sexual health clinic.
Frequently Asked Questions
Birth control pill blood clot risk refers to the chance of developing a blood clot while taking certain hormonal contraceptive pills, especially those containing estrogen. The risk is still low for most people, but it is higher than in people who do not use these pills.
The highest birth control pill blood clot risk is generally associated with combined pills that contain both estrogen and progestin. Progestin-only pills usually have a lower blood clot risk than combined pills.
People who smoke, are over age 35, have obesity, a personal or family history of blood clots, certain inherited clotting disorders, or have recently had surgery may have a higher birth control pill blood clot risk.
Warning signs include leg swelling or pain, chest pain, shortness of breath, sudden severe headache, vision changes, or weakness on one side of the body. These symptoms need urgent medical evaluation.
Birth control pill blood clot risk is uncommon, but it is real. The absolute risk for most healthy users is low, though it is higher than the risk in people who are not using estrogen-containing pills.
Estrogen can increase clotting factors in the blood, which is why pills containing estrogen are linked to a higher birth control pill blood clot risk. The amount of estrogen and the type of progestin can influence the risk.
Birth control pill blood clot risk may be reduced by choosing a non-estrogen method if appropriate, avoiding smoking, managing weight, staying active, and discussing personal risk factors with a clinician before starting a pill.
Progestin-only pills are generally considered to have a lower birth control pill blood clot risk than combined estrogen-progestin pills. They may be a better option for people who should avoid estrogen.
Yes. A blood clot can travel to the lungs and cause a pulmonary embolism, which is a medical emergency. Symptoms can include sudden shortness of breath, chest pain, rapid heart rate, or coughing up blood.
Yes. Blood clots often start in the deep veins of the legs, a condition called deep vein thrombosis. Common signs include swelling, pain, warmth, and redness in one leg.
Do not stop prescription contraception without medical advice unless a clinician tells you to do so. If you have symptoms of a clot, seek urgent care right away; if you are concerned about risk, contact your healthcare professional promptly.
Yes. Birth control pill blood clot risk tends to increase with age, especially in people older than 35 who also smoke or have other risk factors. Age alone is not the only factor, but it can matter.
Yes. Smoking increases cardiovascular risks and can raise the danger of serious complications, including blood clots, especially when combined with estrogen-containing birth control pills.
Pregnancy itself carries a blood clot risk that is often higher than the risk from birth control pills. However, the safest option depends on the individual, and a clinician can help compare risks.
Important factors include smoking, age, obesity, migraine with aura, personal or family history of clots, recent surgery, prolonged immobility, high blood pressure, and known clotting disorders.
Doctors may evaluate symptoms with a physical exam, blood tests, and imaging such as ultrasound for leg clots or CT scans for lung clots. Prompt diagnosis is important because treatment should begin quickly if a clot is found.
The risk itself is not inherited, but some conditions that increase clotting, such as Factor V Leiden or other thrombophilias, can run in families and raise birth control pill blood clot risk.
Yes. Options may include progestin-only pills, hormonal IUDs, copper IUDs, implants, or barrier methods, depending on medical history and personal preferences. A clinician can help choose the safest method.
Blood clot risk can be highest in the first months after starting an estrogen-containing pill, though clots can occur later as well. Any new symptoms should be taken seriously at any time.
Seek emergency help immediately if you have chest pain, trouble breathing, sudden severe headache, fainting, one-sided weakness, or a swollen painful leg while using a birth control pill. These can be signs of a life-threatening clot.
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