Combined oral contraceptives increase blood clot risk primarily because their estrogen component changes the way your liver produces clotting proteins. The risk is small in absolute terms, roughly 6 in 10,000 women per year compared to about 1 to 2 in 10,000 for non-users, but it’s real and varies significantly depending on the type of pill, how long you’ve been taking it, and your individual risk factors.
How Estrogen Shifts Your Clotting Balance
Your blood maintains a careful balance between proteins that form clots and proteins that prevent them. Estrogen, specifically the synthetic estrogen in most combined pills, tips that balance toward clotting. It does this by stimulating your liver to produce more of certain clotting proteins. One key example: Factor VIII, a protein essential for clot formation, increases by roughly 27% within the first days of a pill cycle. At the same time, the natural anticoagulant proteins that normally keep clotting in check, like protein S, can decrease.
The net effect is blood that clots more easily than it should. This doesn’t mean a clot will form, but it means the threshold is lower. If blood flow slows down (from sitting on a long flight, for instance) or a blood vessel wall is irritated, the conditions that would normally resolve harmlessly are more likely to produce a clot that sticks around and grows.
This is why progestin-only pills, which contain no estrogen, don’t carry the same risk. In studies of women who had already experienced a blood clot, the recurrence rate on progestin-only contraceptives was 10.6 per 1,000 women-years, compared to 18.6 per 1,000 for women using no hormones at all. The difference wasn’t statistically significant, meaning progestin-only methods appear clot-neutral. The culprit is specifically estrogen’s effect on the liver.
The First Year Is the Riskiest Window
Blood clot risk is not evenly distributed across the time you spend on the pill. It peaks during the first 12 months, and especially during the first 3 months. Compared to non-users, women in their first year on a combined pill have about 7 times the relative risk of a clot. That drops to 3.6 times for years one through five, and to 3.1 times beyond five years.
This pattern matters for a practical reason: stopping and restarting the pill resets the clock. If you take a break for several months and then start again, you re-enter that higher-risk early window. For this reason, cycling on and off the pill can actually be riskier than staying on it continuously.
Not All Pills Carry the Same Risk
The type of progestin paired with estrogen makes a measurable difference. Pills are loosely grouped by “generation” based on which progestin they contain. Second-generation pills use levonorgestrel, and these carry the lowest clot risk among combined options. Third-generation pills, which contain desogestrel or gestodene, carry about 1.7 times the risk of second-generation pills. Drospirenone, found in several popular brand-name pills, falls into a similar higher-risk category.
Why newer progestins would increase risk more than older ones isn’t fully settled, but part of the explanation involves how different progestins interact with estrogen’s liver effects. Some progestins partially counteract estrogen’s clotting impact, and levonorgestrel appears to do this more effectively than its newer counterparts.
Putting the Numbers in Perspective
The absolute risk of a blood clot on the pill is low. Roughly 6 out of every 10,000 women taking combined pills will develop a clot in a given year. For comparison, pregnancy and the postpartum period carry a rate of about 20 per 10,000 women per year, more than three times higher. The lifetime risk of a blood clot for the general population is about 1 in 1,000.
These numbers mean that for most healthy, non-smoking women under 35, the pill’s clot risk is quite small. But “small” doesn’t mean “zero,” and certain factors can multiply that baseline risk dramatically.
Factors That Multiply Your Risk
Some risk factors interact with oral contraceptives in ways that aren’t just additive but multiplicative. The most striking example involves inherited clotting disorders. Women who carry the Factor V Leiden mutation, a genetic variant present in about 5% of people of European descent, have a baseline clot risk roughly 2.6 times that of non-carriers. But when those women take a combined pill, their risk jumps to approximately 29 to 65 times that of a non-carrier not on the pill, depending on the generation of progestin. Most people don’t know they carry this mutation unless they’ve been specifically tested.
Smoking after age 35 is another major amplifier. Among smokers over 35, cardiovascular deaths attributable to pill use are estimated at about 1 per 10,000 users annually, ten times higher than the rate for smokers under 35. The combination of smoking and oral contraceptive use in this age group creates a steep increase in both clot risk and arterial disease risk. This is why prescribing guidelines classify combined pills as an “unacceptable health risk” for smokers over 35.
Other factors that increase risk include obesity, recent surgery or immobilization, a personal or family history of blood clots, and certain autoimmune conditions like lupus.
Symptoms to Recognize
Blood clots from oral contraceptives most commonly form as deep vein thrombosis (DVT), usually in the legs, or as pulmonary embolism (PE), where a clot travels to the lungs. Knowing the symptoms is important because early treatment drastically improves outcomes.
- DVT signs: pain or tenderness in one leg (often the calf), swelling, warmth, and redness or discoloration of the skin in the affected area.
- PE signs: sudden unexplained shortness of breath, sharp chest pain that worsens with breathing, coughing (sometimes with blood), and fainting.
These symptoms can be subtle at first. A DVT might feel like a pulled muscle. A PE might initially seem like anxiety or a respiratory infection. The key distinguishing feature is that these symptoms tend to appear in one leg (not both) for DVT, and come on suddenly for PE.
Lower-Risk Contraceptive Options
If your risk profile makes combined pills a poor fit, several alternatives avoid the estrogen-driven clotting problem entirely. Progestin-only pills (sometimes called minipills) do not appear to increase clot risk based on available evidence. Hormonal IUDs release progestin locally into the uterus with minimal systemic absorption. Copper IUDs contain no hormones at all. Implants use progestin only.
The CDC’s Medical Eligibility Criteria classify combined hormonal contraceptives as Category 4, meaning “unacceptable health risk,” for women with a history of DVT or PE, known clotting disorders, or certain other conditions. Progestin-only methods generally remain Category 1 or 2 (safe to use) for these same groups, making them a practical alternative for women who need reliable contraception but can’t safely take estrogen.