Some newer progestins used in oral contraceptives can increase a woman's risk of venous thromboembolism by up to 7-fold compared with women who don't take the pills, according to a recent study from Denmark. The research also shows that newer progestins may double the risk of thromboembolism compared with pills that contain the older synthetic progesterone levonorgestrel.
Researchers analyzed Danish national registry data that included 8 million woman-years of observation from 2001 to 2009. They tracked information on how many prescriptions for oral contraceptives were filled, the types of estrogen and progestin they contained, and how long women used the products.
Voelker R. Risk of Blood Clots Higher for Oral Contraceptives With Certain Progestins. JAMA. 2011;306(20):2206. doi:10.1001/jama.2011.1707