HealthDay News — Women who use the combined contraceptive transdermal patch or vaginal ring are at significantly higher risk for venous thrombosis, results of a Danish study show.

Compared with women not using hormonal contraceptives, those who used transdermal combined patches had an almost eight times higher risk of thrombosis (RR 7.9, 95% CI 3.5 to 17.7) and vaginal ring users had a more than six-fold increased risk (RR=6.5; 95% CI: 4.7-8.9), Øjvind Lidegaard, MD, of the University of Copenhagen in Denmark, and colleagues reported in BMJ.

They collected information from national databases and registries on types of contraceptives prescribed to Danish women ages 15 to 49, and cases of venous thrombosis, between 2001 and 2010.

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Among the 1,626,158 women without a history of thrombotic disease and 9,429,128 person-years of follow-up, there were 5,287 cases of venous thrombosis, for a rate of 8.1 per 10,000 person-years.

Among women who did not use hormonal contraceptives incidence of venous thrombosis was 2.1 per 10,000 person years.

During 6,178 person-years of transdermal combination patch use, there were six cases of venous thrombosis, for an adjusted rate ratio of 2.5 (95% CI: 1.1-5.6) compared with women using oral contraceptives.

Thrombotic risk is known to decrease with oral contraceptive use, however, and after adjusting for this, the rate ratio for transdermal patch use was 2.3 (95% CI:1.0-5.2).

The incidence rate was 7.8 per 10,000 years of exposure among women using the combination vaginal ring. When this group of women was compared with those taking oral contraceptives, the rate ratio was 2 (95% CI: 1.4-2.9), and decreased slightly after the researchers adjusted for duration of use (RR=1.9; 95% CI: 1.3-2.7).

Based on these data the researchers estimated that 1,250 women using the patch and 2,000 using the ring would have to switch to oral combined contraceptives, in order to prevent a single venous thrombosis during the course of a year.

“[W]omen are generally advised to use combined oral contraceptives with levonorgestrel or norgestimate, rather than to use transdermal patches or vaginal rings,” the researchers wrote.

Women with subcutaneous contraceptive implants also had an increased relative risk for venous thrombosis compared with those not using hormone contraceptives (RR=1.4; 95% CI: 0.6-3.4). Compared with women using oral contraceptives that contained levonorgestrel, the rate ratio for contraceptive implants was 0.4 (95% CI 0.2 to 1.1).

“The modest non-significant 40% increased relative risk of venous thrombosis in women using subcutaneous implants is not surprising, as other types of progestogen only contraception do not confer an increased risk, and it is less than half the risk found in users of combined oral contraceptives containing levonorgestrel,” the researchers wrote.

For women using the levonorgestrel intrauterine system the relative risk was 0.6 (95% CI 0.4 to 0.8), and the rate ratio compared with women on combined oral contraceptives was 0.2 (95% CI 0.1 to 0.3), suggesting a possible protective effect.

Study limitations included the inability to control for family disposition to venous thrombosis or for BMI, a well documented risk factor for venous thrombosis, the researchers noted.

Lidegaard Ø et al. BMJ. 2012; doi:10.1136/bmj.e2990.