A study published today on bmj.com reports that many women are not using the safest brands of oral contraceptive pill. The report considers the risk of venous thrombosis, such as deep vein thrombosis and pulmonary embolism.
The authors explain: “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.”
A number of large studies since the 1960’s have exposed that the increased risk of deep venous thrombosis connected with oral contraceptive use is two to six times greater. As a consequence, the oestrogen dose in combined oral contraceptives has been reduced. However it is still uncertain which hormonal contraceptive is safest with regard to the risk of venous thrombosis.
In order to find out more, a team of researchers at Leiden University Medical Center in the Netherlands evaluated the thrombotic risk related with presently existing oral contraceptives. Their center of attention was on dosage of oestrogen and type of progestogen.
The results are based on information from a large study including 1,524 women aged 18 to 50 years with a first deep venous thrombosis and 1,760 healthy controls.
Confirming results of earlier studies, they established that women taking oral contraceptives have a five-fold increased risk of venous thrombosis compared with non users.
There were differences in the risk depending on the type of progestogen. For instance, 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 definitely associated with oestrogen dose and was highest during the first three months of use. Regardless of what type of pill was used.
In conclusion the authors argue that the choice of oral contraceptive should be based on the smallest increase of side effects. The safest choice with regard to the risk of venous thrombosis is an oral contraceptive containing levonorgestrel combined with a low dose of estrogen.
A second study also published today, corroborates these findings. Researchers in Denmark examined the risk of venous thrombosis among healthy Danish women aged 15 to 49 years. They were all using different types of hormonal contraception from 1995 to 2005.
Results showed that the risk of venous thrombosis decreased with duration of use and decreasing oestrogen dose. For the same duration of use and the same dosage of oestrogen, oral contraceptives containing levonorgestrel were associated with a significantly lower risk of venous thrombosis than pills containing other types of progestogens.
Pills containing only progestogen and hormone releasing intrauterine devices were not linked with any increased risk of venous thrombosis.
The authors emphasize 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. They recommend a low dose combined pill as first choice for contraception for women of normal weight and without known genetic predispositions.
In an associated editorial Dr Nick Dunn from the University of Southampton remarks: “Despite their different designs, these two studies produce remarkably similar results and confirm past studies of the risk of venous thromboembolism with the pill.” Still, he stresses that the absolute risk of having venous thromboembolism is low, even when taking the pill, and confirms that the products of choice should be those containing either levonorgestrel or norethisterone, with the lowest dose of oestrogen as possible.
A review of information based on evidence on all contraceptive methods presently existing for women is also published on bmj.com.
Names of Combined Contraceptive Pills
“The venous thrombotic risk of oral contraceptives, effects of oestrogen dose and progestogen type: results of the MEGA case-control study”
A van Hylckama Vlieg, research fellow, F M Helmerhorst, professor of clinical epidemiology of fertility,J P Vandenbroucke, professor of clinical epidemiology, C J M Doggen, research fellow, F R Rosendaal, professor of clinical epidemiology, head of department
BMJ 2009; 339:b2921
“Hormonal contraception and risk of venous thromboembolism: national follow-up study”
Øjvind Lidegaard, professor, Ellen Løkkegaard, consultant, Anne Louise Svendsen, statistician, Carsten Agger, data manager
BMJ 2009; 339:b2890
“Oral contraceptives and venous thromboembolism”
Nick Dunn
doi=10.1136/bmj.b3164
“Contraception for women: an evidence based overview”
Jean-Jacques Amy, editor in chief, Vrijesh Tripathi, lecturer
BMJ 2009; 339:b2895
bmj.com
Written by Stephanie Brunner (B.A.)