Researchers say many women are not taking the safest pill brand, after a couple of separate studies in the British Medical Journal revealed some oral contraceptives were linked with a higher blood clot risk than others.

However, experts maintain, whichever brand you use, the risk overall is small as blood clots are a rare side-effect of only a combined pill. The Family Planning Association says, rather than stop taking it, if women are worried about their effect, they should speak to a doctor.

For long, the combined pill containing both oestrogen and progestogen has been associated with an increased risk of venous thrombosis, or a blood clot in the vein, which can prove serious and occasionally fatal, particularly if breaking away it travels to the lungs.

Over the years, oestrogen levels in the pill have been reduced to reduce the risks, however, this risk is far smaller than the risk of a clot during pregnancy.

The first study by Dutch researchers looked at data from 1,524-women with venous thrombosis and found a link in taking the pill to a five-fold increased risk of a clot. However, closer analysis showed variation, as women taking pills containing a progestogen called levonorgestrel e. g. Microgynon, had a four time lower risk of thrombosis, than women not on the pill.

Whereas, women taking contraceptives containing desogestrel e. g. Mercilon or Marvelon, had seven times the risk of those not taking the pill.

Women taking pills with norgestimate e. g. Cilest, had a six-fold extra risk as those on drospirenone e. g. Yasmin, and those taking a drug containing cyproterone acetate e. g. Dianette, often prescribed for acne, had an almost seven-fold additional risk.

Researchers concluded: ‘Currently, available oral contraceptives still have a major impact on thrombosis occurrence, and many women do not use the safest brands with regard to risk of venous thrombosis.’

The second study by Danish researchers also found contraceptives containing levonorgestrel to be associated with a lower risk than those containing desogestrel, gestodene or drospirenone.

It is interesting to note Yasmine, the newest type of pill, Dr Nick Dunn, a GP and senior lecturer at the University of Southampton says, does not offer any advantage over more traditional ones.