Newer kinds of contraceptive pills carry a higher risk of dangerous blood clots, claims a recent study published in the British Medical Journal.
The study compared oral contraceptives containing a synthetic version of the hormone with earlier versions of the pill.
The so-called third-generation pill, introduced in the 1990s, and the fourth generation, approved in the last decade, are designed to skirt side-effects of older types of the contraceptive.
In 2013, European Medicined Agency carried out a safety review after France's medicines agency found the newer pills were linked to a risk of serious blood clot. When a blood clot breaks loose and travels in the blood, it is called a venous thrombo- embolism (VTE).
The new study widens the statistical net, trawling through two large British prescription databases to take into account pill dosage, body fat, smoking and other factors, which affect risk.
Compared with women not using oral contraceptives, women using older pills had about two and a half times increased risk of VTE, it found while women using a newer version of the contraceptive had around a four times increased risk of VTE compared to women who did not take the pill.
There were six extra VTE cases per year per 10,000 women using the newer levonorgestrel and norgestimate pills. This increased to 14 extra cases per 10,000 users of desogestrel and cyproterone.
"Women taking the later-generation contraceptives should not stop using them, but should consult their doctor and review their current type of pill at their next appointment if there are any concerns," said the researchers, led by Yana Vinogradova at the University of Nottingham.