Newer types of birth control pills are more likely to cause serious blood clots than others, according to a study published Tuesday by BMJ (the British Medical Journal).
The authors, led by Dr. Øjvind Lidegaard from the University of Copenhagen, say that women on pills containing one of the newer types of progestogen hormones (drospirenone, desogestrel, or gestodene) have double the risk of blood clots of women on pills containing an older progestogen (levonorgestrel).
Previous studies have indicated that the new types of progestogen hormone might increase clot risk. So Dr. Lidegaard and colleagues carried out a large-scale study to assess the risk for women using oral contraceptives with different formulations.
The researchers reviewed data for all Danish non-pregnant women between the ages of 15 and 49 from January 2001 until December 2009.
The relative risk of blood clots while taking an oral contraceptive pill is still low, even for women taking those with newer types of hormones, explain the authors. Compared with non-users of hormonal contraception, pills with an older progestogen, levonorgestrel, increase the risk of clotting threefold. Pills with the newer hormones -- drospirenone, desogestrel, or gestodene -- increase the risk sixfold.
In absolute terms, the risk of clotting in users of newer pills is about 10 per 10,000 women per year.
Dr. Philip Hannaford from the University of Aberdeen says “many clinicians will choose to minimize the risk by prescribing a combined oral contraceptive with levonorgestrel whenever possible.”
Dr. Hannaford stresses, however, that it is crucial “not to exaggerate the risk – oral contraceptives are remarkably safe and may confer important long term benefits in relations to cancer and mortality.”