"I heard birth control pills can cause blood clots—is it safe?" This is a question that almost never fails to come up when an oral contraceptive consultation begins in the clinic. The word "blood clot" became widely known during the COVID-19 period, which has added to the anxiety. To put the conclusion first, blood clot risk is not a single number that applies equally to everyone—it varies according to each individual's circumstances. That is why a one-time risk assessment and consultation before starting the medication is important. This article summarizes what we check before prescribing and who in particular should be cautious.
What kind of problem is venous thrombosis, and why look into it
Venous thromboembolism is a condition in which a blood clot forms in a deep vein, such as in the leg, and if part of it breaks off and travels to the lungs, it can lead to a pulmonary embolism. Although rare, it can become an emergency when it occurs, so it is a topic that must always be addressed in a birth control pill consultation.
The female hormones in oral contraceptives affect the clotting balance. Platelet activity and the production of clotting proteins and fibrin increase, while the action that dissolves fibrin also increases at the same time; these changes act in combination, and the tendency to clot can rise somewhat. The key point is that the risk is neither zero nor the same for everyone.
If you know that a risk exists, watch for symptoms, and use it under conditions that suit you, oral contraceptives are, for many people, a medication whose benefits outweigh the risks. What matters is a single check before starting.
Risk in numbers, compared with pregnancy
In absolute terms, the blood clot risk of birth control pills is on the small side. According to UK FSRH data, venous thrombosis among women of reproductive age who do not use the pill is reported at the low single digits per 10,000 per year, and somewhat higher with combined hormonal contraceptives.
What is interesting is that pregnancy and childbirth themselves carry a higher blood clot risk than the pill. In FSRH data, the risk during pregnancy is higher than in non-users, and the risk in the postpartum period right after delivery in particular is summarized as much greater. Even so, we do not frighten pregnant women by telling them to "watch out for blood clots." In the clinic, it is not uncommon for people to perceive only the blood clot risk of birth control pills as exaggerated. Understanding the context—that the risk only rises relatively—reduces vague fear.
What we check in the pre-prescription risk assessment
The first step to using it safely is the pre-prescription assessment. FSRH recommendations call for checking the following items before first starting a combined hormonal contraceptive.
- Blood pressure and body mass index (BMI)
- Whether you smoke and how much per day
- Migraine history, especially migraine with aura
- Personal and family history of venous thrombosis
- Whether you are immediately postpartum and whether you are breastfeeding
Based on this information, the clinician judges suitability against standards such as the World Health Organization (WHO) Medical Eligibility Criteria for Contraceptive Use or the UK's UKMEC. These standards grade each condition from category 1 (no restriction) to category 4 (do not use), guiding for whom a combined pill is unproblematic and for whom another method is better. The pre-prescription consultation is the process of applying these yardsticks to the individual. If you are curious about which contraceptive method suits you, you can review your conditions together at the contraception/pregnancy clinic consultation.
Those who especially need caution
Even with the same medication, caution is needed for some people. The cases we look at especially closely in the clinic are as follows.
| Risk factor | Considerations in assessment |
|---|---|
| Aged 35 or older and smoking | May fall into a category where use is not recommended by the standards, so we judge carefully |
| Obesity (high BMI) | The higher the BMI, the higher the category; we view it comprehensively together with other risk factors |
| Personal or family history of blood clots | There may be a hereditary clotting tendency, so additional assessment may be needed |
| Migraine with aura | Treated as a condition requiring separate careful judgment |
In particular, there may be a hereditary tendency to clot, called thrombophilia. Medically known examples include the Factor V Leiden mutation, the prothrombin G20210A mutation, protein C and protein S deficiency, and antithrombin deficiency. Some people live unaware of it, and it is identified during consultation because a family member experienced a clot at a young age. That is why the family history question is not a formality but an actually important clue. For the choice when you have high blood pressure, the choice of oral contraceptives when you have high blood pressure covers it in more detail.
Consult about whether birth control pills are okay for my conditionsHow risk changes with duration and dose of use
Let me point out one common misconception about the duration of use. It is easy to think that the longer you take it, the more the blood clot risk keeps accumulating, but that is not the case. The risk is reported to be relatively high in the first several months of use and then to stabilize. It is known that stopping for more than a month returns you to the initial state, so if you are in a situation where you need to take it consistently, keeping it consistent may be more reasonable than frequently stopping and restarting.
On the dose side, generally the higher the hormone dose, the higher the risk. However, products with high concentrations like the early pills are now hard to find, and most currently on the market are low-dose. In the low-dose range below a certain level, the difference in risk by concentration is summarized as not being distinct. The blood clot risk between oral pills and transdermal forms such as patches is also reported to be not greatly different. If you are looking for a milder ingredient, the explanation of single-ingredient birth control pills is also a useful reference.
How to use it smartly while reducing risk
If you have started the pill, it is good to keep a few things in mind. First, the most important thing is to honestly disclose whether there is any risk factor missed in the pre-start assessment. Second, if symptoms such as swelling and pain in one leg, sudden shortness of breath, chest pain, or a severe one-sided headache occur while taking it, you should seek care without delay.
Third, if a situation where you cannot move for a long time, such as a long flight or major surgery, is scheduled, please let us know in advance. Because it is a period when blood clot risk temporarily rises, we can prepare for it together. If you want to broadly compare which contraceptive method suits you, looking first at the guide to types of contraception is also an option.
In closing
The blood clot risk of oral contraceptives clearly exists, but in absolute terms it is on the small side, and it is reported to be lower than that of pregnancy and childbirth. What matters is, instead of being vaguely afraid or conversely ignoring the risk, to properly check your own conditions once. For smokers aged 35 or older, those with obesity, and those with a personal or family history of blood clots, a pre-prescription consultation is especially meaningful.
If it feels complicated, do not judge alone—we can go over it together.
Written by: Lee Dong-hee Director · OB-GYN Specialist · View medical staff profile
First published December 8, 2023 · Last reviewed May 30, 2026
References: FSRH Combined Hormonal Contraception Guideline (2023), WHO Medical Eligibility Criteria for Contraceptive Use (2015)
This article is intended to provide general health information and does not replace individual diagnosis or treatment. If you have symptoms, please consult through a medical visit.