Published: March 2011

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Combined oral contraceptives and VTE - putting the risk into perspective

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Prescriber Update 32(1): 2
March 2011

The risk of venous thromboembolism (VTE) associated with combined oral contraceptives (COCs) has been widely reported in the medical literature and mainstream media. Prescribers are reminded that this risk needs to be carefully considered in the context of other risk factors for VTE.

The risk of VTE in women is influenced by endogenous and exogenous hormonal exposure, age, family history of VTE, and lifestyle factors such as weight and smoking.

Data compiled by the World Health Organization (WHO) for developed countries in 1997 suggests that that VTE event rate, among non-pregnant women who are not using COCs, increases with age:¹

Age	VTE event rate
20 to 24 years	3.2 events per 100,000 woman-years
30 to 34 years	4.6 events per 100,000 woman-years
40 to 44 years	5.9 events per 100,000 woman-years

The estimated risk of VTE in women who are obese (BMI ≥35 kg/m²) varies but studies demonstrate an up to three-fold increase in risk compared to women of normal weight (BMI <25 kg/m²).^1,2 Current smokers of ≥25g of tobacco per day have up to a two-fold increase in VTE risk compared to those who have never smoked.²

In association with COC use, the risk of VTE is considered to be highest in the first year of therapy. VTE risk also rises with increasing oestrogen dose.³

Epidemiological studies suggest the incidence of VTE in women, with no known risk factors, who use low oestrogen dose COCs (<50 μg) ranges from 20 to 40 cases per 100,000 woman-years. This rate equates to a four to eight-fold increased risk of VTE compared with non-pregnant nonusers for whom an estimated event rate range is 5 to 10 cases per 100,000 woman-years.⁴

The risk of VTE with COCs is significantly less than the risk during pregnancy, which has been estimated at 60 cases per 100,000 pregnancies.³ The risk of VTE is highest in the post-partum period, with an estimated two to five-fold increased risk compared to that during pregnancy.⁵

While it is important to consider the risk of VTE when prescribing a COC, it is also important to consider this risk in relation to other risk factors.

References:

1. Kaunitz A. and Wasthoff C. 2008. Combination hormonal contraception and venous thromboembolism risk. The Journal of Family Practice. 20(8): Suppl.
2. Holst A., Jensen G. and Prescott E. 2010. Risk factors for venous thromboembolism - Results from the Copenhagen City Heart Study. Circulation. 121(17): 1896-1903.
3. Lidegaard Ø., et al. 2009. Hormonal contraception and risk of venous thromboembolism: national follow-up study. British Medical Journal. 339(7720): 557-560.
4. MHRA. 2010. Yasmin: Update on risk of venous thromboembolism. Drug Safety Update. 3(9): 2-3.
5. Dinger J., Heinemann L. and Kühl-Habich D. 2007. Contraception. 75: 344-354.