The Birth Control Pill and Blood Clots: Real Risk, Real Numbers

Blood clots are the most serious side effect associated with the combined contraceptive pill, and they are the one that tends to generate the most alarm. Stories of young, otherwise healthy women developing deep vein thrombosis or pulmonary embolism while on the pill appear periodically in the press, and they are difficult to read without feeling anxious.

The anxiety is understandable. But most coverage of this topic stops at "the pill raises blood clot risk" and never gets to the number that actually matters: by how much, compared to what, and for whom.

Those are the questions this article answers. The risk is real. The full picture is significantly more complicated.

The Baseline: Blood Clots Without the Pill

The type of blood clot associated with hormonal contraception is venous thromboembolism, or VTE. This includes deep vein thrombosis (DVT), where a clot forms in a deep vein, usually in the leg, and pulmonary embolism (PE), where a clot travels to the lungs. Both are serious and require treatment.

In women of reproductive age who are not using hormonal contraception and are not pregnant, the background incidence of VTE is approximately 2 to 3 per 10,000 women per year. This is sometimes written as 0.02 to 0.03%, which sounds very small. It represents the background rate: the number of clots that occur in this population regardless of any contraceptive use.

Everything else in this conversation is a comparison to that baseline.

Combined Pill: What the Numbers Show

Large Danish registry studies tracking millions of women across multiple years have produced the most comprehensive data available on VTE rates by contraceptive method and progestogen type. The picture they paint is consistent and has been replicated across independent populations.

In absolute terms, the difference between the pill and no pill is approximately 3 to 6 additional cases per 10,000 women per year, depending on the formulation. For someone using levonorgestrel pills: roughly 3 to 4 women out of every 10,000 using that pill in a given year will develop a VTE who would not have developed one without it. For drospirenone pills, that figure is closer to 6.

This is a real risk, and it is not trivial. VTE can be fatal if a pulmonary embolism is large enough or goes untreated. The risk is taken seriously in clinical practice, which is why VTE history, family history of clotting disorders, and conditions like Factor V Leiden are screened for before prescribing combined contraceptives.

But it is a risk that needs to be held alongside other comparisons to be meaningful.

The Pregnancy Comparison

Pregnancy itself is one of the strongest risk factors for VTE in women of reproductive age.

This comparison is not made to dismiss the pill's risk. It is made because it reframes what the pill's risk is being contrasted with. For a person who would become pregnant without the pill, the relevant comparison is not "pill versus no pill in a risk-free world." It is "pill versus pregnancy," and in that comparison, the pill's VTE rate is a fraction of the pregnancy rate.

For people who are not at risk of pregnancy, this comparison is less directly relevant, and the absolute risk from the pill's increment above baseline carries more weight in the decision.

Not All Pills Are Equal: Progestogen Type Matters

One of the most important and underemphasized findings in VTE research is how significantly the risk varies by which progestogen is in the combined pill.

The Lidegaard Danish cohort, and subsequent studies replicating its findings, consistently show that combined pills containing desogestrel, gestodene, or drospirenone carry roughly 1.5 to 2 times the VTE risk of pills containing levonorgestrel, when controlling for age, BMI, and other confounders. In absolute terms, this translates to a difference of approximately 3 to 5 additional cases per 10,000 women per year between the highest-risk and lowest-risk combined pill progestogens.

This does not mean that desogestrel or drospirenone pills are dangerous for most people. For someone without VTE risk factors, the absolute risk remains low regardless of progestogen type. But for someone with elevated baseline risk, whether from family history, obesity, immobility, or a known clotting tendency, the choice of progestogen is a meaningful clinical variable worth discussing with a prescriber.

Progestin-Only Pills: A Genuinely Different Risk Profile

This distinction is important and is often lost in coverage of pill-related VTE risk. When a headline says "the pill raises blood clot risk," it is almost always referring to the combined pill. The progestin-only pill is a genuinely different situation.

The reason is mechanistic. The VTE risk associated with combined pills is driven primarily by the estrogen component. Estrogen stimulates the production of certain clotting factors in the liver and affects platelet aggregation in ways that tip the coagulation balance. Progestin-only pills contain no estrogen, and their systemic progestin levels are lower than the progestin dose in combined pills. Without the estrogen contribution, the clotting risk does not materialize in the same way.

For someone who has a personal or strong family history of VTE and wants hormonal contraception, progestin-only options, particularly the progestin-only pill or the hormonal implant, are the standard clinical alternative to combined pills.

Personal Risk Factors That Change the Calculation

VTE risk is not uniform across all pill users. Several factors substantially modify the calculation and should be part of any pre-prescription conversation.

• Inherited clotting disorders Factor V Leiden mutation and prothrombin gene mutation are the two most common inherited thrombophilias. Women carrying Factor V Leiden who use combined oral contraceptives have a VTE risk estimated at 15 to 35 times higher than women without either condition, depending on whether they are heterozygous or homozygous for the mutation. This is one of the clearest cases where combined pill use is contraindicated, and taking a thorough family history before prescribing is standard clinical practice in many countries.
• Obesity A BMI above 30 is independently associated with elevated VTE risk, and the pill's risk increment adds to this baseline rather than replacing it. For people with obesity, the absolute risk of VTE on the combined pill is meaningfully higher than for people without this additional risk factor.
• Smoking and age The combination of smoking, combined pill use, and age over 35 significantly elevates overall cardiovascular risk. Combined pill use is generally not recommended for smokers over 35, primarily because of arterial event risk (stroke, heart attack), but the combination is also relevant in the VTE context.
• Immobility Prolonged immobility, such as on long-haul flights, reduces venous blood flow and is an independent VTE risk factor. The advice to move regularly on long flights applies to everyone, but is particularly relevant for combined pill users who have other risk factors. This does not mean pill users should avoid flying; it means compression, hydration, and movement matter more.
• Previous VTE A personal history of DVT or PE is a contraindication to combined pill use in most clinical guidelines. If you have had a VTE, combined oral contraceptives are not appropriate, and progestin-only alternatives or non-hormonal methods should be used instead.

Recognising Symptoms

Knowing the risk is one thing. Knowing what to look for is another.

At a Glance: VTE Rates by Situation

Rates shown are approximate figures from population-level studies. Individual risk depends on personal factors including age, BMI, smoking status, family history, and inherited clotting disorders.

The Full Picture

The combined pill does raise VTE risk, and that is not in dispute. The increase is real and it matters for clinical decisions.

What the full picture shows: the absolute risk for most pill users without additional risk factors is low, the risk varies significantly by progestogen type, progestin-only pills do not carry meaningful VTE risk, and the alternative of pregnancy carries VTE risk substantially higher than any oral contraceptive.

For people with individual risk factors that elevate their baseline, a proper clinical assessment before starting the combined pill is important, not because the pill is uniquely dangerous, but because individual risk profiles differ significantly. The right contraceptive for someone with Factor V Leiden is not the same as the right one for someone without it, and that distinction deserves a proper conversation with a prescriber rather than a one-size-fits-all answer.