"Third generation" oral contraceptives are linked to a more than doubled risk of potentially fatal blood clots known as venous thromboembolisms.

The research from Boston University School of Medicine in Massachusetts indicates that oral contraceptives containing desogestrel or gestodene increased the risk of blood clots by a factor of 2.3 compared to older birth control pills containing levonorgestrel.

The findings support earlier research about the potential danger of the "third-generation" pills that caused a scare in 1995 and resulted in a warning to doctors and pharmacists about the potential dangers. Since the warning, the number of women taking the "third generation" pill, have dropped by about 80%.

The researchers estimated that if women had not switched after the warning there would have been about 26% more cases of blood clots.

In an emailed response to the study, Anthony Griffiths points out that the "third-generation" pills might me of use to women in certain situations, such as women suffering from the androgenic effects of polycystic ovarian disease syndrome (PCOS) and women at an increased risk of heart disease.

Women with PCOS would benefit from the greater estrogenic effects of the "third generation" pills.

Women at increased risk of heart disease would be better off avoiding the levonorgestrel pills which can lower the levels of cardio-protective HDL cholesterol.

British Medical Journal November 11, 2000;321:1190-1195