Which combined oral contraceptive pill has the lowest risk of pulmonary embolism, ischemic stroke and myocardial infarction?

Millions of women worldwide receive oral contraceptive pills for birth control or to control excessive menstrual bleeding. Use of oral contraceptives is associated with a small but important increased risk of pulmonary embolism, as well as stroke and myocardial infarction.

This study of more than 5 million women looked at the risk of developing these thrombotic events in association with 8 different types of combined oral contraceptive pills available in France. The estrogen component was the same (ethinyl estradiol), but in doses varying from 20 to 50 ug and the progestin component was 1 of 4 different types.

The investigators made 2 important observations:

1. Oral contraceptive pills with a higher estrogen dose (30 to 40 µg) were associated with an increased risk for thrombosis compared with lower estrogen doses (20 µg).
2. The risk of pulmonary embolism was higher when low-dose estrogen was combined with desogestrel or gestodene compared with levonorgestrel.

The first observation supports what others have already observed about the dose-dependent association of risk of thrombosis with estrogen. The second observation suggests that while estrogen is key to thrombotic risk, the type of progestin may also be a contributing factor.

There are important limitations to this population-based cohort study that must be considered. Data on why a particular oral contraceptive pill was selected and on additional risk factors such as family history of thrombosis or smoking were not available in the database. Furthermore, the study duration was likely too short to capture arterial thrombotic events such as myocardial infarction. More important, deep vein thrombosis (DVT) was not included as an outcome because the database included only hospital-based events. Failure to collect these data should not affect the comparisons between drugs, but it will have resulted in an underestimation of the overall incidence of venous thromboembolism because DVT is diagnosed more often than PE.

Despite these limitations, this large study provides useful estimates of the thrombotic risk for combined oral contraceptives and suggests that the overall thrombotic risk is small (ranging from 7 to 33 in 100,000 women taking combined oral contraceptive pills for 1 year). It also confirms the association between oral contraceptive pill–associated thrombosis and age: About 1 in 4 thrombotic events occurred in women over 35 years of age.

Doctor, I have heard that the birth control pill (oral contraceptive pill) can cause blood clots. Is it safe for me to take one?

The risk of forming a blood clot when taking an oral contraceptive pill is low. About 1-2 out of 10,000 women who take the pill for 1 year will be diagnosed with a blood clot in their lungs or have a heart attack or stroke. To keep this risk as low as possible, we recommend you start with the pill containing the lowest dose of hormones, particularly estrogen.

However, it is important to understand that the risk of blood clots while taking the pill maybe higher if you are over 35 years of age, smoke, or have family members who have had blood clots. Women who have these additional risk factors may wish to discuss their options with a gynecologist.