Tens of thousands of women should consider switching their contraceptive pill to reduce the risk of a potential blood cot, new research suggests.
Women taking so-called "third generation" contraceptive pills, those developed in the 1980s, are up to three times more likely to suffer a blood clot than those who use older varieties, according to new research based on British and American records.
With no real other advantages of the newer drug, scientists said it would be "prudent" to make the older pills the "first line choice".
Scientists say doctors should prescribe pills that contain the hormone levonorgestrel, rather than drospirenone.
Dr Susan Jicks, of Boston University School of Medicine, and colleagues carried out two studies into the two types of birth control drugs and found the difference in threat was dramatic.
They said their findings "provide further evidence that levonorgestrel oral contraceptives appear to be a safer choice".
Around three million women in Britain are thought to take the pill to prevent pregnancy, and 100 million worldwide.
While all types of the oral contraceptive increase the risk of a clot, some are worse than others.
The debate first started in October 1995 when scientists warned women taking the third generation pill such as Yasmin which has around 750,000 prescriptions issued a year – were at greater risk of developing blood clots in the deep veins of the legs or the pelvis.
Called venous thromboembolism (VTE), the clots can be fatal if they move inside the circulatory system and lodge in the lungs.
In the first study, based on US medical claims data, the researchers found a twofold increased risk of a non-fatal blood clot in women using pills containing the hormone drospirenone compared with women using the older ones which have levonorgestrel.
This risk remained even after taking account of other possible causes.
The actual incidence rates were 30.8 for every 100,000 women years using the drospirenone pill and 12.5 per 100,000 women years using the levonorgestrel pill.
The second study, using data from the UK General Practice Research Database, found a threefold increased risk of a first non-fatal blood clot in women using drospirenone pills oral compared with those on the levonorgestrel ones.
The actual rates for blood clots in this study were 23 per 100,000 women years using the drospirenone pill and 9.1 per 100,000 women years using the levonorgestrel pill.
The researchers said: "These findings support more recent studies that suggest that drospirenone oral contraceptives are not as safe as levonorgestrel oral contraceptives with respect to venous thromboembolism and, in the absence of other considerations, should not be the first choice in oral contraception."
They added: "Perhaps now is the time for a systematic review on this topic.
"In the meantime, as no clear evidence exists to show that the use of the drospirenone pill confers benefits above those of other oral contraceptives in preventing pregnancy, treating acne, alleviating premenstrual syndrome, or avoiding weight gain, prescribing lower risk levonorgestrel preparations as the first line choice in women wishing to take an oral contraceptive would seem prudent."
The study was published online in the British Medical Journal.
The Family Planning Association advised women to see their GP if they are worried about the contraception they are taking.