Two of the most popular birth control pills available in Canada — Yasmin and Yaz — carry a higher risk of serious blood clots than some other oral contraceptives, two new studies say.

The studies — both published Thursday in the online edition of the British Medical Journal — show that pills containing a new type of progestogen hormone called drospirenone carry up to triple the risk of serious blood clots (known as venous thromboembolisms) over an older generation of pills containing a hormone called levonorgestrel.

Drospirenone is the key ingredient in Yasmin and Yaz, which are manufactured by Bayer and widely prescribed in Canada.

Drospirenone-based pills like Yasmin and Yaz had worldwide sales of $2 billion US in 2009, reports indicate.

CBC's Marketplace program raised health concerns about Yasmin and Yaz in January. Co-host Erica Johnson talked to a number of young women who have taken the two brands of pills and experienced serious health problems.

More than 4,000 women have taken legal action against Bayer and more than 800 in Canada have joined a class-action lawsuit.

Older pills 'safer'

The findings provide further evidence that the older oral contraceptives "appear to be a safer choice with regard to venous thromboembolism than preparations containing drospirenone," says Dr. Susan Jick of Boston University's School of Medicine, who led the research team.

In the first study, researchers found the risk of a non-fatal blood clot among women using drospirenone-based pills was double the risk for women using levonorgestrel-containing oral contraceptives.

The study points out that the overall risk of serious blood clots remains low — even among women taking the drospirenoane-based birth control pills. The incidence rates were 30.8 per 100,000 woman years among the drospirenone users versus 12.5 per 100,000 woman years among users of levonorgestrel-based pills.

Researchers studied existing U.S. medical claims data to arrive at their findings.

In the second study, researchers found the risk of a first non-fatal blood clot almost tripled among women using drospirenone-containing oral contraceptives versus women who used the older levonorgestrel-based pills.

"Prescribing lower risk levonorgestrel preparations as the first-line choice in women wishing to take an oral contraceptive would seem prudent," the authors say.

In a statement, Bayer Canada said "the manner in which the authors applied the study methodology reported in these two publications show significant flaws" and maintained that previous studies have shown that Yasmin and Yaz are as safe as other birth control pills.

"Given the already large and robust scientific body of evidence, in Bayer’s opinion, these studies do not change the overall assessment about the safety of Bayer’s oral contraceptives," the company said.

With files from the CBC's Erica Johnson