Blood clots in women on the Pill are rare
Many women are not taking the safest brand of the pill, say researchers.
Two separate studies in the British Medical Journal found that some oral contraceptives were linked with a higher risk of blood clot than others.
But experts stressed that blood clots are a rare side-effect of the combined pill and the risk overall is small, whichever brand is used.
Women should not stop taking it but speak to a doctor if they are worried, the Family Planning Association said.
It has been known for a long time that the combined pill, which contains both oestrogen and progestogen, was associated with an increased risk of venous thrombosis - a blood clot that forms in a vein.
In some cases a clot can be serious and occasionally fatal, particularly if it breaks away and travels to the lungs.
The levels of oestrogen in the pill have been reduced over the years to help cut the risks.
However, this risk is far smaller than the risk of a clot during pregnancy.
In the first study done by Dutch researchers - looking at data from 1,524 women who had developed venous thrombosis - they found that overall taking the pill was associated with a five-fold increased risk of a clot.
But closer analysis showed variation.
Women taking pills containing a progestogen called levonorgestrel (for example, Microgynon) had the lowest risk of thrombosis at four times that of women not on the pill.
Whereas those on contraceptives containing desogestrel (for example, Mercilon or Marvelon) had the highest risk, at seven times that of those not taking the pill.
Women taking a pill with norgestimate (for example, Cilest) had an almost six-fold extra risk as did those on drospirenone (for example, Yasmin).
Those taking a drug containing cyproterone acetate (for example, Dianette, which is often prescribed for acne) had an almost seven-fold additional risk.
"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 researchers concluded.
The second study, by Danish researchers, also found that contraceptives containing levonorgestrel were associated with a lower risk than those containing desogestrel, gestodene or drospirenone.
Dr Nick Dunn, a GP and senior lecturer at the University of Southampton said it was interesting that Yasmine, the newest type of pill, did not offer any advantage over more traditional ones.
But he added that those recommended by the researchers were probably the most commonly prescribed.
"What this says is we should stick to prescribing the well-trusted favourites but the chance of having a blood clot when on the pill are very low anyway."
He said if a woman wanted a particular brand maybe because of personal experience he would still prescribe it.
Lynn Hearton, from the Family Planning Association (FPA), said: "Although the combined pill does slightly increase the risk of thrombosis, the risk is still really low.
"If any women are worried about the pill they should not stop using it.
"They should continue taking it and seek advice from a health professional."