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Higher blood clot risk found for newer unsubsidised oral contraceptive pill

Students taking notes in a lecture

Friday 22 April 2011 8:52am

New evidence that a combined oral contraceptive pill containing the progestogen hormone drospirenone appears to put women at higher risk of blood clots than older types containing levonorgestrel has just been published by researchers at the University of Otago and Boston University.

In a study appearing in the British Medical Journal (BMJ), the researchers found that UK women currently using the drospirenone pill were about three times more likely to develop non-fatal venous thromboembolism than similar women taking levonorgestrel formulations.

Study co-author Dr Lianne Parkin of the University of Otago’s Department of Preventive and Social Medicine says that venous thromboembolism is a rare but potentially life-threatening condition involving blood clots in the legs (deep vein thrombosis or DVT) which sometimes break off and travel to the lung (pulmonary embolism).

“In absolute terms, we found that the risk of venous thromboembolism was about 23 per 100,000 ‘woman-years’ in users of the drospirenone pill and 9 per 100,000 woman-years in users of levonorgestrel pills,” Dr Parkin says. “In other words, among women taking the drospirenone oral contraceptive, each year an extra 14 women per 100,000 users suffered a venous thromboembolic event which could have been avoided if they had been taking a levonorgestrel pill instead.”

“Previous studies have found that the so-called third-generation oral contraceptives, and those containing the progestogen cyproterone acetate, are associated with a higher risk of venous thromboembolism than levonorgestrel formulations. Our study adds to emerging evidence that the drospirenone pill also carries a higher risk,” she says. “However, unlike the third-generation and cyproterone acetate oral contraceptives, the drospirenone pill has never been subsidised in New Zealand, so it is not widely used here.”

The researchers used data from the UK General Practice Research Database to undertake a study based on 318,825 women aged 15 to 44 years who used drospirenone or levonorgestrel oral contraceptives at any time between 2002 and 2009. Women with major risk factors for thromboembolism were excluded from the study and influences such as body mass index and smoking were taken into account in the analyses.

“Our findings of an increased thromboembolism risk were consistent with results from two previous epidemiological studies by Danish and Dutch researchers. Also, laboratory studies have shown that the use of drospirenone, third-generation, and cyproterone acetate pills have a greater impact on a particular aspect of blood clotting (activated protein C resistance) that increases the risk of thromboembolism than do preparations containing levonorgestrel.”

Dr Parkin says that currently there is no clear evidence that drospirenone pills have any greater clinical benefit than levonorgestrel pills in preventing pregnancy, treating acne, alleviating premenstrual syndrome, or avoiding weight gain.

For more information, contact

Dr Lianne Parkin
Department of Preventive and Social Medicine
University of Otago

The study can be found at the BMJ web site as one of its open access research articles

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