Contraceptive Pills and Risk of Blood Clots

The combined oral contraceptive pill is one of the most widely used forms of birth control in the world, taken by an estimated 150–200 million people. It is highly effective, well studied, and for most people, very safe. But one risk comes up more than any other in consultations: blood clots.

Here our clinical team explains what the actual risk is, who is most affected, and what warning signs to look out for — so you can make an informed choice rather than a fearful one.

How the contraceptive pill works

Hormonal contraceptives prevent pregnancy using synthetic versions of the hormones oestrogen and progesterone. Most people take a combined pill, containing both. The oestrogen component suppresses the pituitary gland's release of follicle-stimulating hormone (FSH) and luteinising hormone (LH), which prevents an egg from maturing and being released. The progestogen component thickens cervical mucus and thins the womb lining, adding two further layers of protection.

There is also the progestogen-only pill (sometimes called the mini-pill), which contains no oestrogen at all — a distinction that matters a great deal when discussing clot risk.

Why oestrogen affects clotting

Oestrogen stimulates the liver to produce more of the clotting proteins involved in coagulation, while slightly reducing the body's natural anticoagulant activity. The net effect is blood that clots a little more readily. This is why concerns about venous thromboembolism (VTE) — clots in the deep veins of the leg, or in the lungs — have followed the combined pill since it was first introduced in the 1960s.

Importantly, this effect is driven by oestrogen. Progestogen-only pills are not associated with a meaningful increase in VTE risk, which makes them a common alternative for people who cannot take oestrogen.

How big is the risk, really?

Context matters enormously here, because the absolute numbers are small:

• In women not using hormonal contraception and not pregnant, roughly 2 in 10,000 will have a VTE in a year
• In women taking a combined oral contraceptive, this rises to roughly 5 to 12 in 10,000 per year, depending on the specific progestogen used
• During pregnancy, the rate is around 29 in 10,000, and in the weeks immediately after giving birth it is higher still — several times greater than on the pill

In other words, the pill does raise the relative risk, but from a very low baseline, and pregnancy itself carries a considerably higher risk than the contraception used to prevent it.

Who is at higher risk?

Certain factors compound the risk and are the reason a clinician reviews your full history before prescribing. Combined hormonal contraception may not be suitable if you:

• Have had a previous blood clot (DVT or pulmonary embolism), or have a first-degree relative who had one at a young age
• Have a known inherited clotting disorder, such as Factor V Leiden or a prothrombin gene mutation
• Smoke and are aged 35 or over
• Have a BMI of 35 or above
• Get migraine with aura
• Have uncontrolled high blood pressure, or certain heart or liver conditions
• Are immobilised long-term, or are due to have major surgery

Research using large patient databases has found that women with a history of venous thrombosis face a substantially multiplied risk when taking combined contraceptives — which is why previous clots are usually an absolute contraindication rather than a judgement call.

Warning signs to act on

Seek urgent medical attention if you experience:

• Pain, swelling, warmth or redness in one calf or thigh
• Sudden breathlessness, chest pain that worsens on breathing in, or coughing up blood
• Sudden severe headache, weakness or numbness on one side, or difficulty speaking
• Sudden loss of vision

These can indicate a deep vein thrombosis, pulmonary embolism or stroke, and require immediate assessment — call 999 or go to A&E.

Reducing your risk

If combined contraception is appropriate for you, a few things help:

• Stop smoking — the single biggest modifiable factor
• Stay mobile on long flights or journeys; walk about and stay hydrated
• Tell your clinician before any planned surgery or period of immobility
• Attend your contraception reviews so your blood pressure and risk profile are reassessed
• Report any new migraine with aura promptly

Getting contraception with Rightangled

Contraception is a prescription-only medicine and is never a one-size-fits-all decision. At Rightangled, every request is reviewed by our clinical team, which includes GPhC-registered independent prescribers, with medical oversight from our doctor, Dr Abdullah. Your medical history, blood pressure, BMI, smoking status and family history are all assessed before anything is issued — and if a combined pill is not suitable for you, an oestrogen-free alternative may be discussed.

You can verify our credentials at any time: we are registered with the General Pharmaceutical Council (GPhC registration 9011933), certified by LegitScript, and independently reviewed on Trustpilot.

You may also find our article on genetics and blood clots useful if clotting runs in your family.

This article is for general information and does not replace personalised medical advice. If you are concerned about your health, speak to a clinician.