Obese women taking the combined pill are staggeringly 24 times more likely to develop life-threatening blood clots, according to new research. And the risk is still 12 times in those who are just overweight in taking the pill that contains oestrogen and progestin.
The combined pill contains a synthetic version of oestrogen - a hormone which can cause venous thromboembolism (VTE). The condition reduces blood flow to organs - potentially triggering heart attacks or strokes.
Lead author Professor Giuseppe Rosano, of the IRCCS San Raffaele Pisana, Rome, said: "It is well established both obesity and oestrogen-containing contraceptives are risk factors for VTE. Despite this, obese women continue to receive these drugs.
"The scientific evidence indicates obesity and combined oral contraceptives have a synergistic effect on VTE risk and this should be considered in prescribing decisions. "
He advised them to use the 'mini-pill' - which only has chemicals which mimic the hormone progestin - or other methods. Both prevent pregnancy.
Women who have had breast cancer, have previously suffered blood clots or have a family history of either are already precluded from the combined pill. Prof Rosano said: "Progestin-only products, including pills, intrauterine devices or implants are a safer alternative to the combined pill in women carrying excess weight."
The study in ESC (European Society of Cardiology) Heart Failure is based on a review of the latest evidence. It found a combination of overweight or obesity and the combined oral contraceptives increases the danger of blood clots in women of reproductive age.
In the UK, more than half women of childbearing age are overweight - with one in five obese. The Italian team found they are nearly twice and two-and-a-half times more likely to develop blood clots, respectively.
However, in combined pill users, the risk rose 12 and 24 times respectively when compared with non users with a BMI (body mass index) in the normal range.
Prof Rosano said: "This article highlights the latest evidence on the independent effects of obesity and contraceptives, and their synergistic effects, on VTE risk and provides clinical recommendations. VTE refers to a blood clot in a vein and includes two life-threatening conditions - deep venous thrombosis and pulmonary embolism."
The World Health Organisation estimates global prevalence of obesity nearly tripled between 1975 and 2016 - with one-in-seven women being obese.
Risk of VTE increases progressively with BMI - more than doubling in obese women. The biggest impact occurs in under 40s - soaring fivefold.
Prof Rosano said: "The particularly high risk in obese women under 40 is important, since it is at this age that many seek contraception."
Overall, the combined pill is linked to increasing the likelihood of VTE three to seven times. There is no link between the mini-pill and blood clots.
Prof Rosano said: "Obese women taking contraceptives are vulnerable to VTE and should take steps to limit their other predisposing factors for cardiovascular disease - for example by quitting smoking and increasing their physical activity levels."
Drug company bosses have been accused of holding back the British launch of a pioneering pill that could transform birth control. The medication, Slynd, contains progestogen only, causes fewer side effects and is more effective than the mini-pill.
It can be used at any time during a 24-hour period. Others have to be taken during a short window every day.
The revolutionary tablet has been widely available in America and Australia since 2019 and received its UK licence in March last year from the Medicines and Healthcare Products Regulatory Agency. But manufacturer Exeltis is not expecting it to be available in the UK until at least next year.
Financial watchdog NICE (National Institute for Health and Care Excellence) will have to approve prescription on the NHS.
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