A study published online in the British Medical Journal confirms previous findings that certain oral contraceptive（口服避孕药） pills are more likely to cause serious blood clots (venous thromboembolism -- VTE静脉血栓栓塞 ) than others. 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（孕激素） hormone (drospirenone, desogestrel or gestodene) have double the risk of VTE than women on pills containing an older progestogen (levonorgestrel).

Previous studies have indicated that the new types of progestogen hormone might increase the risk of VTE. So Lidegaard and colleagues carried out a large-scale study to assess the risk of VTE for women using oral contraceptives with different progestogens.

The researchers reviewed data of the hormonal contraception patterns and first time VTE episodes for all Danish non-pregnant women between the ages of 15 and 49 from January 2001 until December 2009.

The participants had no previous record of either blood clots or cancer before the study began.

The research team assessed over eight million women years of observation and during this period there were 4,246 first episodes of VTE.

The relative risk of VTE whilst taking the oral contraceptive pill is still low, explain the authors. Compared with non-users of hormonal contraception, pills with levonorgestrel increase the risk of VTE threefold and pills with drospirenone（黄体素） , desogestrel or gestodene increase the risk sixfold.

In absolute terms, the risk of VTE in current users of newer pills is about 10 per 10,000 women years. This means that about 2,000 women should shift from using oral contraceptives with desogestrel, gestodene, or drospirenone to those with levonorgestrel to prevent one event of VTE in one year, say the authors.

The increased risk remained even after taking account of other possible causes for VTE, they conclude.

In an accompanying editorial, Dr Philip Hannaford from the University of Aberdeen says "it is difficult not to conclude that combined oral contraceptives with desogestrel, gestodene or drospirenone confer a higher risk of venous thromboembolism than those with levonorgestrel" and that "many clinicians will choose to minimize the risk by prescribing a combined oral contraceptive with levonorgestrel whenever possible."

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."