A new study found that using oral contraceptives substantially reduces women’s risk of getting ovarian cancer: the longer they use them, the less likely they are to develop the disease, concluded the researchers.
The study is published in the 26th January issue of the The Lancet and was conducted by Professor Valerie Beral and a large number of co-researchers and collaborators from the Collaborative Group on Epidemiological Studies of Ovarian Cancer. Professor Beral is based at the Cancer Research UK Epidemiology Unit, Oxford University, UK.
Scientists have known for a long time that using the oral contraceptive pill is linked to reduced incidence of ovarian cancer, but the eventual public health effects depend on how long the protection lasts after women stop taking the pill.
Oral contraceptives have been in use for nearly 50 years, and are taken by over 100 million woman worldwide.
In this study, Beral and colleagues analysed and checked pooled data from 45 epidemiological studies of ovarian cancer conducted in 21 countries. This gave them individual data from 23,257 women with ovarian cancer (the cases), and 87,303 women without ovarian cancer (the controls).
7,308 (31 per cent) of the women with ovarian cancer, and 32,717 (37 per cent) of the women without ovarian cancer, had a history of oral contraceptive use.
The mean age of diagnosis in women with ovarian cancer was 56 years, and the median year of diagnosis was 1993. The average length of time for which women in the ovarian cancer group had used oral contraceptives was 4.4 years, compared with 5.0 years in the control group.
The results showed that:
- The longer that women used oral contraceptives, the bigger the reduction in risk of ovarian cancer.
- The risk reduction persisted for more than 30 years after women stopped taking the pill, but got less as time went on.
- The proportional risk reduction for every 5 years of use was 29 per cent up to 10 years of stopping use, 19 per cent for 10 – 19 years after stopping, and 15 per cent for 20 – 29 years after stopping.
- Proportional risk reductions for use during the 1960s, 1970s and 1980s were similar, even though typical oestrogen doses in the 60s were more than double those of the 80s.
- One type of ovarian cancer, mucinous tumours (12 per cent of total incidence), appeared not to be affected by use of oral contraceptives, but otherwise there was little variation in risk reduction among cancer types.
- In high income countries, using oral contraceptives for 10 years was linked to reduced rate of ovarian cancer before the age of 75 from 12 to 8 per 1,000 women.
- This was also linked to a reduction in rate of death from 7 to 5 per 1,000 women.
- Every 5,000 woman-years of oral contraceptive use appears to prevent 2 cases of ovarian cancer and one death before the age of 75.
- Risk reduction did not vary much with respect to women’s ethnicity, education, onset of menstruation, family history of breast cancer, use of hormone replacement therapy, BMI (body mass index), height, or use of alcohol or tobacco.
5,000 woman-years of oral contraceptive use is for example 5,000 women using the pill for a year, or 500 women for 10 years. This kind of statistic is useful for looking at risk reductions on a population wide basis, to inform a public health view of a disease, for example.
Beral and colleagues concluded that:
“Use of oral contraceptives confers long-term protection against ovarian cancer.”
They added that the findings suggested:
“Oral contraceptives have already prevented some 200,000 ovarian cancers and 100,000 deaths from the disease, and that over the next few decades the number of cancers prevented will rise to at least 30,000 per year.”
An accompanying editorial that discusses the overall risk-benefits of oral contraceptives, calls for contraceptive pills to be available to women over the counter because the case is now so convincing.
And in an accompanying comment article, Dr Eduardo Franco, McGill University, Montreal, Quebec, Canada, and Dr Eliane Duarte-Franco, Institut National de Santé Publique de Québec, Montreal, Quebec, Canada, said that:
“As for the link between oral contraceptives and ovarian cancer, today’s collaborative analysis brings unequivocal good news. Women and their health-care providers are once again at a balancing act of judging risks versus benefits.”
“Ovarian cancer and oral contraceptives: collaborative reanalysis of data from 45 epidemiological studies including 23,257 women with ovarian cancer and 87,303 controls.”
Collaborative Group on Epidemiological Studies of Ovarian Cancer.
The Lancet 2008; 371:303-314.
DOI:10.1016/S0140-6736(08)60167-1
Volume 371, Number 9609, 26 January 2008.
Sources: The Lancet press release and journal article.
Written by: Catharine Paddock