Many women are not using the safest brands of oral contraceptive pill with regard to the risk of venous thrombosis (deep vein thrombosis and pulmonary embolism), finds a study published on bmj.com today.
The risk differs by type of progestogen and dose of oestrogen, and the safest option is an oral contraceptive containing levonorgestrel combined with a low dose of oestrogen, say the authors.
Since 1961, several large studies have shown a twofold to sixfold increased risk of deep venous thrombosis associated with oral contraceptive use. As a result, the oestrogen dose in combined oral contraceptives has been reduced. But it is still unclear which hormonal contraceptive is safest with regard to the risk of venous thrombosis.
So a team of researchers at Leiden University Medical Center in the Netherlands assessed the thrombotic risk associated with currently available oral contraceptives. Their focus was on dose of oestrogen and type of progestogen.
The findings are based on data from a large study of 1,524 women aged 18-50 years with a first deep venous thrombosis and 1,760 healthy controls.
In line with results of previous studies, they found that women taking oral contraceptives have a five-fold increased risk of venous thrombosis compared with non users.
This risk differed by type of progestogen. For example, pills containing desogestrel were associated with a twofold increased risk of venous thrombosis compared with pills containing levonorgestrel. The risk of venous thrombosis was also positively associated with oestrogen dose and was highest during the first three months of use, irrespective of the type of pill used.
The authors conclude that the choice of oral contraceptive should be based on the smallest increase of side effects and, as such, the safest option with regard to the risk of venous thrombosis is an oral contraceptive containing levonorgestrel combined with a low dose of estrogen.
These findings are supported in a second study, also published today. Researchers in Denmark assessed the risk of venous thrombosis among healthy Danish women aged 15-49 years who were using different types of hormonal contraception from 1995 to 2005. A total of 10.4 million woman years were recorded and 4,213 venous thrombotic events were observed.
They found that the risk of venous thrombosis decreased with duration of use and decreasing oestrogen dose. For the same dose of oestrogen and the same length of use, oral contraceptives containing levonorgestrel conferred a significantly lower risk of venous thrombosis than pills containing other types of progestogens.
Progestogen only pills and hormone releasing intrauterine devices were not associated with any increased risk of venous thrombosis.
The authors stress that the absolute risk of venous thrombosis with use of any types of combined oral contraceptives in young women is less than one in 1,000 user years. And for women of normal weight and without known genetic predispositions, they recommend a low dose combined pill as first choice for contraception.
Despite their different designs, these two studies produce remarkably similar results and confirm past studies of the risk of venous thromboembolism with the pill, says Dr Nick Dunn from the University of Southampton in an accompanying editorial. However, he points out that the absolute risk of having venous thromboembolism is low, even when taking the pill, and agrees that the products of choice should be those containing either levonorgestrel or norethisterone, with as low a dose of oestrogen as possible.