London: Women using newer contraceptive pills may double their risk of developing dangerous blood clots compared to those who use older types, researchers say.

Scientists have already warned that all types of oral contraceptive pills slightly increase the risk of developing clots, which can lead to deep vein thrombosis in the leg or arm and a pulmonary embolism -- a potentially fatal blockage in the blood supply to the lungs.

However, a US study, which included data from British general practices, claimed that there is more than double the risk depending on the type of the hormone ‘progestogen’ used in the product, a daily reported.

The study by researchers from the Boston University School of Medicine found a twofold increased risk in non-fatal blood clots in women taking a drospirenone-containing pill, such as 'Yasmin', compared to the levonorgestrel-based pill -- Microgynon, for example.

If 100,000 women were on the newer contraceptive pill for a year, there would be 30 such events of blood clots, compared with 12.5 on the older type.

In another analysis, using GP data from the UK, the study also found a considerably increased risk for women taking drospirenone-containing brands compared to those using levonorgestrel-containing pills.

They reported 23 cases of blood clots per 100,000 women taking drospirenone-containing brands, compared to nine cases for those using older pills.

Writing in the British Medical Journal, the researchers said that levonorgestrel-containing pills were a safer choice’.

However, Dr Anne Szarewski, editor-in-chief of the Journal of Family Planning and Reproductive Heath Care, said a major European study found no difference in thrombosis rates between different types of Pills.

She said: "All Pills carry a risk but it is misleading to suggest some are less safe than others. "A family history of blood clots is most important in assessing a woman’s risk factors but she may be too young for this to have become apparent.

"Other research also stresses the importance of smoking, which doubles the risk, and being overweight."

According to the UK's National Prescribing Centre, all combined oral contraceptives increase the risk of venous thrombosis.

But, an international panel of experts in 2009 found blood clots caused by the pills were a "rare event", with the highest risk in the first few months of use which falls away to similar levels seen in "non-Pill" users.

(Agencies)