Risk of venous thromboembolism among users of third generation oral contraceptives compared with oral contraceptives with levonorgestrel before and after the 1995 warning: cohort and case-control analysis

A study in this week's BMJ finds that the third generation oral contraceptive pills are associated with around a two-fold increased risk of clots in the veins. These findings are consistent with previous studies, but conflict with a study published recently in the BMJ surrounding the 1995 'pill scare.'

Using data from UK general practices between January 1993 and December 1999, researchers at Boston University in the USA evaluated the overall incidence and risks of venous clotting in users of third generation oral contraceptive pills and oral contraceptive pills containing the hormone levonorgestrel, before and after the scare. They found that the risk of venous clotting is around twice as high among users of third generation pills than it is among users of pills with levonorgestrel, and the difference is significant.

They also found that a shift away from the use of third generation pills after the scare was more pronounced among younger women (who have a lower risk of venous clotting) than among older women. Had the use of third generation pills remained unchanged after the warning, the authors estimated that about 26% more cases of venous clotting would have occurred. If more older women had shifted away from use of third generation pills, the authors suggest there would have been an even greater decrease in venous clotting.

Several reasons may explain why these conclusions differ from the recent study by Farmer and colleagues, say the authors. These include the methods used to exclude women from the analysis who were at risk of clots for other reasons, and taking into account other risk factors for clots, such as obesity and smoking.

Contact:

Hershel Jick, Boston University School of Medicine, Lexington, USA
Tel: 1-781-862-6660
Fax: 1-781-862-1680 Email: [email protected]