Women who are smokers and use hormonal contraceptives – whether pills, patches or rings – are at greater risk for blood clots and heart attacks, according to the Spanish Heart Federation.

A study by the University of Copenhagen published in the British Medical Journal and carried out on 9.4 million women between the ages of 15 and 49, found that users of some method of hormonal contraception are at greater risk for blood clots than women who don’t use them.

The data from the study showed that these illnesses are eight times more common among users of contraceptive patches, 6.5 times more common among users of vaginal rings, and 3 times higher among those who take oral contraceptives.

Another study also published in the British Medical Journal and carried out by the UK General Practice Research Database on women between the ages of 15 and 44 showed that blood clots are more frequent among women who take contraceptives with Drospirenone – a synthetic form of progesterone – than among those who use contraceptives containing levonorgestrel, which is an older type of synthetic progesterone.

“These studies show that the use of hormonal contraceptives is not free of secondary effects, especially when it comes to cardiovascular health,” said Almudena Castro, president of Rehabilitation and Preventive Cardiology at the Spanish Cardiology Society.

“Consequently, women should be warned of their potential risks, especially women who are at increased risk of blood clots.”

“However,” Castro added, “each individual case should be analyzed when prescribing hormonal contraceptives, as a healthy 24-year-old smoker will be different that a 40-year-old overweight smoker.”

Hormonal contraceptives are known to have the potential to cause blood clots both in veins and in arteries. They also affect lipids in the blood and increase the concentration of cholesterol and triglycerides.