Scientists have found new evidence to prove that contraceptive pills these days could increase risk of clot formation in women.

The results show that pills containing one of the newer types of progestogen hormone
(drospirenone, desogestrel, gestodene, and cyproterone) are associated with an increased risk of VTE than pills containing older progestogens (levonorgestrel and norethisterone). The University of Nottingham researchers said that this was “an important clarifying study” that “has sufficient power to provide reliable comparative findings for different formulations of combined oral contraceptives.”

The researchers, led by research fellow Yana Vinogradova, used prescription data from two large UK general practice databases to measure the associations between use of combined oral contraceptives and risk of VTE in women aged 15-49 years, adjusting for other known risk factors.

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. Women using newer pills, containing drospirenone, desogestrel, gestodene, and cyproterone, had around a four times increased risk of VTE. Risks for women using newer pills were around 1.5-1.8 times higher than for women using older pills.

The authors stressed that oral contraceptives have been remarkably safe, and point out that the reported three times increased risk of VTE in women using oral contraceptives in their study was still lower than the up to 10-fold increased risk of VTE in pregnant women. They say 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.”

This is an observational study, and therefore, no definitive conclusions can be drawn about cause and effect.