A study recently published in the British Medical Journal shows that the safety of some oral contraceptives, with regards to their risk of causing pulmonary embolism, stroke or heart attack, depends on the quantity of estrogen and type of progestogen they contain
Combined oral contraceptives, or birth control pills, are taken by millions of women across the world. These pills contain a combination of two hormones called estrogen and progestogen. These hormones work together to stop ovulation and cause changes in the uterine wall that prevent pregnancy as well as to regulate the menstrual cycle and improve acne problems. Most combined birth control pills contain the same two ingredients however many different combinations are available. These mixtures vary based on the amount of estrogen as well as the type of progestogen they contain. While there are many benefits connected to the use of birth control pills, the safety of oral contraceptives appears to fluctuate from combination to combination.
One of the more serious potential side effects of oral contraceptive use is the risk of a thromboembolism. This occurs when a blood clot forms somewhere within the body and a piece breaks off causing the blockage of a blood vessel. These blockages can occur in vessels that support the lungs, brain and heart leading to events known as pulmonary embolisms, strokes, and heart attacks, respectively. These blockages cause tissue damage which, in turn, can result in serious injury or even death. Although the occurrence of these side effects as a result of birth control use are relatively rare, appearing in only 0.06% of the women included in this study, it is important to understand the factors that may increase the risk of these events.
Recently, a research group in France has published an article in the British Medical Journal in which they set out to determine the combined oral contraceptive that confers the lowest risk of pulmonary embolisms, strokes and heart attacks. They started by identifying 14.8 million women of child bearing age in the French national health Insurance database (SNIIRAM). The information contained here allowed the investigators to determine which women had received reimbursement for oral contraceptive prescriptions. Additionally, through an anonymous number on their health insurance card, these women were linked to the French hospital discharge database (PMSI). This enabled the investigators to monitor which women were hospitalized as a result of a pulmonary embolism, stroke or heart attack. All women with a history of these events were ineligible for the study.
During the two years in which the study was conducted, nearly 5 million French women were found to be of child bearing age, had no history of thromboembolisms and were reimbursed for oral contraceptive pills by the French national health insurance. In France, eight different combined oral contraceptives under 26 brand names are eligible for such reimbursement. These combinations include both low and high doses of estrogen, designated by the investigators as 20mg and 30-40mg, respectively, as well as five different types of progestogen: levonorgestrel, desogestrel, gestodene, norgestrel and norethisterone. This study compared the incidence of thromboembolisms in the groups of women taking these eight different combined birth control pills to determine which confer higher or lower risk of adverse events. Women were removed from the study if at any point they stopped taking oral contraceptives, became pregnant, experienced a pulmonary embolism, stroke or heart attack, reached age 50, or died.
Consistent with the findings of other analyses, this study found that risk of pulmonary embolism is increased in women taking oral contraceptives containing higher doses of estrogen. In particular, when controlling for type of progestogen, taking birth control pills containing 20 mg of estrogen compared with 30-40 mg decreased risk of pulmonary embolism by 25%, stroke by 18%, and heart attack by 44%. Interestingly, in addition to this discovery, the researchers found that combined oral contraceptives containing levonorgestrel and 20 mg of estrogen conferred the lowest risk of thromboembolism compared with the other combinations.
In France, the most common birth control prescription is levonorgestrel combined with 30 mg of estrogen while the study presented here shows that pills containing levonorgestrel and 20 mg of estrogen confer a significantly lower risk of thromboembolism. Findings like this have the potential to guide administration of the safest oral contraceptives for women, however, the authors of the paper suggest exerting caution before jumping to conclusions. They indicate that, although their results have the potential to guide these decisions in the future, further validation is required before they can definitively say which oral contraceptive confers the lowest risk of pulmonary embolism, stroke and heart attack.
Written By: Katrina Cristall, BSc (Hons)