Blood clots from the Pill studied

22^nd April 2011 - Two new studies report that some types of contraceptive pills increase the risk of women suffering serious blood clots, though the overall risk is still small.

The research, published online at bmj.com found that pills containing the newer type of progestogen hormone, drospirenone, carry up a two to three times increased risk of venous thromboembolism than pills containing the older progestogen, levonorgestrel.

The new research adds to previous studies on combined oral contraceptives containing desogestrel, gestodene and cyproterone acetate linking them with a higher risk of venous thromboembolism than levonorgestrel contraceptives.

UK & US data

One study used data on women aged 15-44 from the UK General Practice Research Database. Ruling out other possible causes, it found a three times increased risk of a first non-fatal venous thromboembolism in women using pills containing drospirenone compared with women using the Pill containing levonorgestrel.

Another study using US medical claims data found the risk was double.

The limitations of the UK data were the small number of clot cases. The US data didn't allow access to primary medical notes, or take smoking into account.

Although the risks were higher with drospirenone, the actual clot rates were relatively small - 23 to 30.8 per 100,000 women years compared to 9.1 to 12.5 per 100,000 women years using the levonorgestrel pill.

However, the research team from Boston University School of Medicine write that the findings "provide further evidence that levonorgestrel oral contraceptives appear to be a safer choice with regard to venous thromboembolism than preparations containing drospirenone."

Next steps & reaction

The authors say more research is needed in this area, but suggest that "prescribing lower risk levonorgestrel preparations as the first line choice in women wishing to take an oral contraceptive would seem prudent."

A spokesperson from the health charity Brook told us in an emailed statement: "The contraceptive pill is one of the most effective and safe forms of contraception and women should be given impartial information and advice to help them make an informed choice about the type of contraception they would like to use.

"The overall risk of DVT with contraceptive pills still remains lower than in pregnancy. Brook always urges women concerned about the pill not to stop taking their pill immediately and risk an unplanned pregnancy, but to visit their GP, Brook or family planning clinic to discuss the concerns."