Risk Of Venous Thrombosis With Oral Contraceptives

Studies have shown a slight increase in the risk of deep vein thrombosis (DVT) for women on oral contraceptives. The absolute risk of venous thromboembolism is small, ranging from 10 to 30 cases per 100 000 women per year in women using oral contraceptives, versus 4 per 100 000 in nonpregnant women not using oral contraceptives. The risk in pregnancy is much greater, estimated to be 60 per 100 000 pregnant women. Women travelers on oral contraceptives should be advised about this small risk and taught preventative measures such as exercise and hydration. They can also decrease their risk by avoiding smoking, maintaining a normal blood pressure and normal weight. A woman with a personal history of a venous thrombosis should not take oral contraceptives. A common scenario may involve a woman traveler with a strong family history of venous thrombosis who wants to take oral contraception for an expedition to a remote area with no access to medical care. In this case she might be screened for one of the biochemical or genetic defects associated with an increased risk of venous thrombosis to get a better idea of her actual risk; for example, factor V Leiden mutation or protein C, protein S or antithrombin deficiencies have been associated with venous thrombosis (Vandenbroucke et al., 2001).

Some studies have found no difference in risk for venous thrombosis between the low-dose second- and third-generation oral contraceptive pill formulations containing less than 50 ^g ethinylestradiol (Farmer et al., 2000). Newer studies on the effects of the second- and third-generation oral contraceptives show a net pro-

thrombic effect. See the article by Vanderbroucke et al. (2001) for an excellent review of the recent data and theories describing possible hemostatic mechanisms relating to the risk of venous thrombosis associated with the use of oral contraceptives. These authors also discuss the pros and cons of screening for genetic defects associated with an increased risk of clotting.

For travel to altitude above 4000 m the risk-benefit ratio should be discussed, as should the possibility of using another method. If a woman insists on continuing her pill and she has no known risk factors she should maintain hydration and takes an aspirin a day. Of the thousands of woman students on oral contraceptives seen over a period of 10 years in one advice center, and trekking to over 4000 m, not one case of DVT was seen. A well-designed study to address this issue needs to be undertaken.

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