Women who use some non-oral forms of hormonal contraception are at higher risk of blood clots than those not taking any hormone-based birth control, a Danish study suggests.

In Friday's issue of the British Medical Journal, Professor Øjvind Lidegaard at the University of Copenhagen reviewed data on risks of the patches and rings for venous thrombosis among all Danish non-pregnant women aged 15 to 49 from 2001 to 2010.

During the study period, the researchers found 3,434 confirmed diagnoses of venous thrombosis — a clot blocking a vein or artery.

While the risks of blood clots in women using birth control pills containing drospirenone came to light in 2010, the risks of contraceptive skin patches and vaginal rings haven't been studied as much.

When a clot forms in the arteries supplying the heart, and blood flow is blocked, it may result in a heart attack. If a clot forms in the arteries supplying the brain, blocking blood flow, it may cause a stroke.

The risk of venous thrombosis among women age 15-49 years who did not use any type of hormonal contraception was on average two events per 10,000 years of exposure.

The researchers concluded 2,000 women using the vaginal ring and 1,250 using the transdermal patch should shift to combined oral contraceptives containing levonorgestrel to prevent one event of venous thrombosis in one year.

They called the increased risk for contraceptive implants "modest."

Using a levonorgestrel-only intrauterine device did not raise the risk and may have a protective effect, the study's authors said.

Investigators took age and education level into account in their statistical analysis.

But they did not have information on body mass index. Body fat is a known risk factor for the clots.

The lead author has received fees from contraceptive maker Bayer Pharma Denmark, MSD Denmark, and Theramex.

Expenses for the study were covered by the Gynaecological Clinic at Juliane Marie Centre in Rigshospitalet in Copenhagen.