(HealthDay News) – Compared to nonusers of hormonal contraception, women who use combined contraceptive transdermal patches are at an eight-fold increased risk of venous thrombosis, and vaginal ring users are at a more than six-fold increased risk, according to a study published online May 10 in BMJ.

Øjvind Lidegaard, MD, from the University of Copenhagen in Denmark, and colleagues analyzed data from four national registries on 1,626,158 non-pregnant Danish women, aged 15–49 years, who were free of previous cancer or thrombotic disease.

The researchers found that, over the nine-year study period, there were 3,434 confirmed first-ever venous thrombosis events. The incidence rate of confirmed events was 2.1 per 10,000 woman years in nonusers of hormonal contraception. After adjusting for other variables, the relative risk of confirmed venous thrombosis was 7.9 (95% CI, 3.5–17.7) in users of transdermal combined contraceptive patches and 6.5 (95% CI, 4.7–8.9) in users of the vaginal ring, compared to nonusers. There was also increased relative risk in women who used subcutaneous implants (1.4; 95% CI, 0.6–3.4), but not in those who used the levonorgestrel intrauterine system (0.6; 95% CI, 0.4–0.8). Compared to users of combined oral contraceptives containing levonorgestrel, the adjusted relative risk of venous thrombosis in transdermal patch and vaginal ring users was 2.3 (95% CI, 1–5.2) and 1.9 (95% CI, 1.3–2.7), respectively.

“Women who use transdermal patches or vaginal rings for contraception have a 7.9 and 6.5 times increased risk of confirmed venous thrombosis compared with nonusers of hormonal contraception of the same age,” the authors conclude.

Several authors disclosed financial ties to the pharmaceutical industry.

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