A study published on the British Medical Journal website adds to the evidence that certain non-oral hormonal contraceptives (e.g. skin patches, implants and vaginal rings) carry a higher risk of serious blood clots (known as venous thromboembolism) than others.

The findings suggest that some women should switch from a non-oral product to a contraceptive pill to help reduce their risk.

Several studies have assessed the risk of venous thrombosis (a collective term for deep vein thrombosis and pulmonary embolism) in women using oral contraceptive pills, but few studies have assessed the risk in users of non-oral hormonal contraceptives.

These products more continuously release hormones into the body to prevent pregnancy.

A team, led by Professor Øjvind Lidegaard at the University of Copenhagen, reviewed data on non-oral hormonal contraceptive use and first ever venous thrombosis in all Danish non-pregnant women aged between 15 and 49 years from 2001 to 2010. All the women had no record of either blood clots or cancer before the study began.

Several factors that could affect the results, including age and education level, were taken into account.

The results are based on 9,429,128 observation years during which 3,434 confirmed diagnoses of first ever venous thrombosis were recorded.

The risk of venous thrombosis among women who did not use any type of hormonal contraception and who were 15-49 years old was on average two events per 10,000 exposure years. Women taking a combined oral contraceptive pill containing the hormone levonorgestrel had a three times increased risk (6.2 events per 10,000 exposure years).

Compared with non-users of the same age, women who used a skin patch had an eight times increased risk (9.7 events per 10,000 exposure years), while women who used a vaginal ring had a 6.5 times increased risk (7.8 events per 10,000 exposure years).

Use of a progestogen-only subcutaneous implant carried a slightly increased risk, while use of a progestogen-only intrauterine device did not confer any risk, and may even have a protective effect, say the authors.

Unlike combined pills, no reduction in risk was seen with long-term use of a patch or a vaginal ring.

Based on these findings, the authors calculated that 2,000 women using a vaginal ring and 1,250 women using a skin patch should shift to a combined pill containing levonorgestrel to prevent one event of venous thrombosis in one year.