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Women who take a new generation of birth control pills, such as Yaz and Yasmin, are two to three times more likely to develop blood clots than those who use older products like Alesse and Aviane, new research shows.

Oral contraceptives all contain a synthetic form of estrogen (ethinyl estradiol) and a synthetic form of progesterone called progestin; the latter comes in various forms that distinguishes products.

Oral contraceptives contain synthetic forms of two female sex hormones, estrogen and progesterone. The latter comes in various forms, including drospirenone (found in new products) and levonorgestrel (used in the older generation of pills).

A pair of studies, published in Friday's edition of the British Medical Journal, show that birth control pills that contain drospirenone carry up to a threefold increased risk of a condition called venous thromboembolism than pills containing levonorgestrel.

Susan Jick, a professor of epidemiology in the School of Public Health of Boston University and principal author of one study, said the findings "provide further evidence that levonorgestrel oral contraceptives appear to be a safer choice."

She and her team examined the records of all U.S. women aged 15 to 44 who filed an insurance claim for a prescription for birth control pills between 2002 and 2009.

The rate of non-fatal blood clots was 30.8 per 100,000 among those using a pill containing drospirenone and 12.5 per 100,000 among those taking a pill containing levonorgestrel.

A second study, which examined the prescription records of British women produced similar numbers. There, the rate of blood clots was 23 per 100,000 in those taking products like Yaz and 9.1 per 100,000 in women using a pill with a formulation like Alesse.

Lianne Parkin, a senior lecturer in the School of Medicine at the University of Otago in Dunedin, New Zealand and senior author of the second paper, said "prescribing lower-risk levonorgestrel preparations as the first line choice in women wishing to take an oral contraceptive would seem prudent."

André Lalonde, vice-president of the Society of Obstetricians and Gynaecologists of Canada, said the new research would be examined carefully, but cautioned against jumping to conclusions.

"Each time a new product comes out, we see this - it's the new pill effect," he said. "Risks and complications are always greatest in new patients, much higher than those who have used the same product for a long time."

Women choose contraceptives based on various criteria, including side effects, price and secondary benefits other than birth control. For example, products like Yaz and Yasmin are reputed to reduce the symptoms of pre-menstrual syndrome, while other formulations are said to reduce acne.

Dr. Lalonde said that while blood clots can occur with all oral contraceptives, the risk is low and needs to be kept in perspective. "We can't forget that birth control pills are taken for a reason - to prevent pregnancy. The risk of blood clots is much higher in pregnancy than from taking these products," he said.

During pregnancy, estrogen and progesterone levels change and blood clots more easily; oral contraceptives provide women with hormones that imitate pregnancy and thus have the same effect.

Yaz and Yasmin are marketed by Bayer Inc. of Toronto. Bayer faces class-action lawsuits in both Canada and the U.S. from women who claim they were not adequately warned of higher risks of developing blood clots and gallbladder disease. The allegations have not been proven in court.

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