Does the Birth Control Pill Raise the Risk of Blood Clots?

Yes, the birth control pill can raise the risk of blood clots, and the effect is linked mainly to pills that contain estrogen (combined pills). For most healthy people the added risk is small, but it is higher with smoking, migraine with aura, high blood pressure, older age, or a personal or family history of clots. This article explains why, who may be affected, which warning signs need 911, and what to discuss with your clinician.

Does the birth control pill raise the risk of blood clots?

According to MedlinePlus (NIH), on its Birth Control page, combined hormonal pills, which contain both estrogen and a progestin, are associated with a higher chance of developing a blood clot than not using hormonal birth control, although the absolute risk remains low. The product labeling for combined pills carries a boxed warning that cigarette smoking increases the risk of serious cardiovascular events, particularly in women over 35 who smoke, so clinicians ask about smoking before prescribing them.

The increase for any one person is usually small. A clot is still uncommon in people who take the pill. But because the pill is used by so many people, clinicians take the risk seriously and screen for factors that add to it.

What kind of clot are we talking about?

A blood clot forms when blood thickens into a clump inside a vessel. Several kinds matter with hormonal medicines:

• Deep vein thrombosis (DVT): a clot in a deep vein, most often in a leg. It can cause swelling, pain, warmth or redness in one leg.
• Pulmonary embolism (PE): a piece of a clot travels to the lungs. It can cause sudden shortness of breath, chest pain, or coughing, and can be life-threatening. MedlinePlus (NIH) describes these symptoms on its Pulmonary Embolism page.
• Stroke or heart attack: a clot in an artery of the brain or heart. These are less common in younger people taking the pill, but the risk can rise with smoking, high blood pressure and migraine with aura.

MedlinePlus explains more about how clots form and how they are treated on its Blood Clots page.

Why would estrogen affect clotting?

Estrogen can change levels of several proteins in the blood that help it clot, and it may also reduce some of the body's natural clot-dissolving activity. The full mechanism is not completely understood, and several factors contribute. The type of progestin and the estrogen dose in a pill may also influence risk, which is why different pills are not all treated as identical.

Who may have a higher risk on the pill?

Estrogen adds to risks you already carry. Tell your clinician if any of these apply so they can decide which method is appropriate for you:

• A past clot in a leg, lung, brain or heart, or a close relative who had a clot at a young age.
• An inherited clotting condition, such as factor V Leiden. Many people do not know they have one until a clot occurs.
• Smoking, especially at age 35 or older.
• Migraine with aura, meaning visual or other sensory changes before the headache. MedlinePlus has background on Migraine. Aura is associated with a higher stroke risk, and estrogen can add to it, so clinicians generally advise against combined hormonal pills for people with migraine with aura. New aura while taking an estrogen-containing pill is a warning sign to raise with your prescriber the same day.
• High blood pressure. See MedlinePlus on How to Prevent High Blood Pressure.
• Recent surgery, long immobilization, a leg cast, or a long period of bed rest.
• Recent childbirth. Clotting risk is higher after delivery, so clinicians often time the start of estrogen-containing methods with care.
• Older age and higher body weight, which are each associated with greater clot risk.

Some people take the pill for reasons other than contraception, for example polycystic ovary syndrome, endometriosis or painful, heavy or irregular periods. The same risk screening applies. Your clinician can weigh the benefit for your condition against the clot risk.

Is pregnancy safer or riskier than the pill?

Pregnancy and the weeks after delivery are themselves associated with a higher clot risk than the usual baseline. Clinicians generally compare the pill's added risk with that background, not with zero. This comparison is one reason many people with no risk factors still use the pill after discussing it with a clinician.

Are there options without estrogen?

Yes. Methods that contain only a progestin or no hormone at all are generally not linked to the same estrogen-related clot concern. Examples include progestin-only pills, hormonal and copper IUDs, the implant, and condoms. Each has its own side effects and trade-offs. MedlinePlus has an overview on Birth Control that compares methods. Some progestin-only injections have their own cautions, so ask your prescriber about the specific product.

What to tell your clinician before starting the pill

• Any past clot, or a relative with a clot, stroke or heart attack at a young age.
• Smoking or vaping habits.
• Migraine, and whether you see flashing lights, blind spots or zigzag lines first.
• Blood pressure readings, diabetes, or high cholesterol.
• Upcoming surgery or travel that involves long immobility.
• All other medicines and supplements you take.

Clinicians often use contraceptive eligibility checklists that match methods to health conditions. Your clinician combines your history, an examination and, when needed, testing to decide what is appropriate.

Warning signs of a clot while on the pill

If you already take the pill, know the symptoms. A clot can progress quickly, so a possible clot needs urgent evaluation.

• One leg that is swollen, painful, warm or red, especially if it started on one side only.
• Sudden shortness of breath, sharp chest pain that worsens with a deep breath, or coughing up blood.
• A sudden severe headache unlike your usual ones, or sudden vision loss, trouble speaking, face drooping, or weakness or numbness on one side.
• Crushing chest pain or pressure, with sweating, nausea or pain spreading to the arm or jaw.

Stroke signs and symptoms of a clot in the lung go to 911 first. Do not drive yourself. One-sided leg swelling, pain, warmth or redness needs same-day medical evaluation. Visual aura that is new for you, without weakness, numbness, speech trouble or face drooping, calls for contacting your prescriber the same day. If aura comes with any of those stroke signs, call 911.

Should you stop the pill on your own?

If you have warning signs of a clot, get emergency care (call 911 for the signs above) and tell the team you take a hormonal birth control pill. If you only have concerns about your risk, talk to your prescriber before stopping, because stopping without another method in place can lead to an unplanned pregnancy, which carries its own clot risk. Your prescriber can help you switch safely and choose a replacement method.

Ways to lower your overall clot risk

• If you smoke, ask your clinician about help quitting. Smoking adds to the clot and cardiovascular risk that estrogen carries.
• On long trips, walk and stretch regularly and stay hydrated.
• Tell any surgeon or hospital team that you take a hormonal medicine, so they can decide whether to adjust it before the procedure.
• Keep up with blood pressure checks.

Where other hormonal medicines fit

Estrogen taken for other reasons, such as menopausal hormone therapy, carries related concerns, so mention any hormone product you use. If you ever need treatment for a clot, your clinician will choose a blood thinner suited to you. Some antibiotics and other medicines can interact with the pill, so ask your pharmacist or prescriber about backup contraception whenever you start a new medicine.

When to Seek Medical Care