Users of any combined oral contraceptive are at an increased risk of the serious blood clots known as venous thromboembolism (VTE) compared with non-users of similar age and health. Photo: Tim Ireland/PA Wire

There is a link between newer contraceptive Pills and a higher risk of serious blood clots, according to a study.

Nottingham University researchers, who describe the results published in the BMJ as “an important clarifying study”, found that users of any combined oral contraceptive are at an increased risk of the serious blood clots known as venous thromboembolism (VTE) compared with non-users of similar age and health.

Pills containing one of the newer types of progestogen hormone - such as drospirenone, desogestrel, gestodene, and cyproterone - are associated with an increased risk of VTE than pills containing older progestogens such as levonorgestrel and norethisterone, it was found.

The researchers claim the risks for women using newer Pills were around 1.5-1.8 times higher than for women using older Pills.

Compared with women not using oral contraceptives, women using older pills, containing levonorgestrel, norethisterone, and norgestimate, had about two-and-a-half times increased risk of VTE, according to the researchers.

Women using newer Pills, containing drospirenone, desogestrel, gestodene, and cyproterone, had around a four times increased risk of VTE.

The study looked at the prescription information from two large UK general practice databases involving women who were aged 15-49 years. They wanted to look deeper into the well-known risk of increased clotting that has been associated with combined oral contraceptives.

The researchers argued that the results “provide evidence for relevant authorities concerned with prescribing guidelines or those involved with regulation of safety of medicines”.

About nine per cent of women of reproductive age worldwide use oral contraceptives, rising to 18 per cent of women in developed countries.

The researchers stressed that oral contraceptives are safe and that women on combined contraceptive drugs “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”.

No final conclusions can be made about the reasons for this newfound link but the researchers believe the study “has the statistical power and sufficient adjustment” to help produce reliable estimates of possible risk.

Helen Stokes-Lampard of the Royal College of GPs, described the combined Pill as an “excellent contraceptive choice for the majority of women”.

She suggested that women already taking the Pill should not change or stop their contraceptives, adding: “As with any prescribed drug, there is always the possibility of negative side effects but GPs understand the risks associated with the Pill - and different generations of the Pill - and will take these into account, along with medical and family history, before prescribing it to their patients.

“The safety of our patients is always paramount for GPs when making decisions about prescribing.

“When starting a woman on oral contraception, prescribers should remember that the second generation combined oral contraceptive pill is associated with a slightly lower risk of complications of thromboembolism than the newer, third generation pills.

“But this must be kept in perspective as the newer pills have some other advantages over second generation pills in terms of fewer side effects, so a decision specific to that patient's needs and her previous experiences should be made.

“If women do have questions about their contraception, there is no need to panic and they should just raise these when they next visit their family planning clinic or GP surgery.”