Friday January 25 2008
Many women take oral contraceptive pills
Taking the contraceptive pill can cut the risk of ovarian cancer, reported The Sun and other newspapers. The protection lasts “30 years after a woman stops taking it”, the newspaper said. The Guardian reported that the pill “has prevented about 200,000 cases of ovarian cancer and 100,000 deaths around the world”.
The newspaper reports are based on a large review of many studies involving 110,000 women which found that taking the oral contraceptive pill for 15 years halves the risk of ovarian cancer. This review does not investigate how the pill works to decrease the risk of ovarian cancer, but other research has suggested that suppressing ovulation in women (as the pill does) may have a beneficial on ovarian cancer risk.
Sir Richard Peto, one of the authors of the study, says that this reduction is bigger than any increase in risk for other types of cancer that have been linked to the Pill, and that the lives of 200,000 users may have been spared. This reliable study published by a world-renowned group of scientists provides convincing evidence that oral contraception protects against ovarian cancer.
Where did the story come from?
The research was coordinated by the Collaborative Group on Epidemiological Studies of Ovarian Cancer, a large international group of experts co-ordinated by a writing and steering group based in Oxford. Collection and analysis of the data was supported by Cancer Research UK and the Medical Research Council. It was published in the (peer-reviewed) medical journal: The Lancet.
What kind of scientific study was this?
This was a meta-analysis of data from 45 epidemiological studies. The researchers analysed data from 45 studies of ovarian cancer, involving 23,257 women with cancer and 87,303 healthy women, from 21 countries. The data were combined statistically to estimate a woman’s chance (relative risk) of developing ovarian cancer if she had taken the oral contraceptive pill containing the hormone oestrogen (the Pill), compared with women who had never taken the Pill. The researchers looked at the data separately for different age groups, for women with different numbers of pregnancies and according to whether they had had a hysterectomy or not.
The researchers found 13 cohort studies that had followed women over time (prospectively) and 32 case-control studies where women who had already developed ovarian cancer and a control group who had not, were asked detail about whether they had taken the Pill in the past (retrospectively). Of these case-control studies, 19 used women from the general population as controls (without cancer) and 13 used women of a similar age from hospital groups as their control. Statistical techniques were used to ensure that women in one study were only compared to similar women in the same study. Cohort studies were only included if data had been collected for more than 100 women with cancer and case-control studies required 40 women. Some of the studies were published in the 1970s and the date of diagnosis for women in the studies ranged from 1973 to 2001.
What were the results of the study?
Overall 7,308 (31 per cent) of the 23,257 women who developed ovarian cancer and 32,717 (37 per cent) of the 87,303 healthy “control” women had used oral contraceptives at least once. The average length of time they took the Pill was similar in both groups (4.4 years for those developing cancer and five years in the control group). The average year of cancer diagnosis was 1993, when the women were, on average, of 56-years-old. When they were compared with women who had never taken the Pill, those women who had ever taken the Pill were about a third less likely to have developed ovarian cancer.
When the researchers looked at the link between use of the Pill and the risk of ovarian cancer, they found that the longer a woman took the Pill, the lower her risk of ovarian cancer. This difference was statistically significant. This reduction in risk persisted for more than 30 years after the women had stopped taking the Pill, but it reduced over time. There was a reduction in risk for every five years of use and in women who were currently using or who had been off the pill for less than 10 years the reduction was 29 per cent. This fell to 19 per cent for women who had last used the Pill 10–19 years previously and to 15 per cent for women who had last used the Pill 20–29 years previously. This suggests that the benefits of taking the Pill last for a long time. The largest effect was nearly a 50 per cent reduction.
The researchers took into account the fact that the dose of oestrogen in a typical pill had halved between the1960s and 1980s and they looked to see if analysing the data by year made a difference to the result. They found that it did not, i.e. that use during the 1960s, 1970s and 1980s was associated with similar proportional risk reductions in ovarian cancer.
These proportional risk reductions were then related to the actual number of women developing ovarian cancer and the number of cancers that potentially could be prevented by taking the Pill were calculated. In high-income countries, 10 years of oral contraceptive use was estimated to reduce the number of ovarian cancers before age 75 years from 12 to eight per 1,000 women using the Pill and the death rate from seven to five per 1,000 women taking the Pill.
Women take the Pill for different lengths of time and so, to take this into account, the researchers sum up the data by saying “for every 5,000 woman taking the Pill for a year, about two ovarian cancers and one death from the disease before 75 are prevented.”
What interpretations did the researchers draw from these results?
The researchers say that “use of oral contraceptives confers long-term protection against ovarian cancer”. They use their data to estimate the potential benefits of the Pill in terms of the numbers of ovarian cancers prevented worldwide. They suggest that “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”.
What does the NHS Knowledge Service make of this study?
This is a large study collecting data from a number of observational studies looking at the rates of ovarian cancer. The methods used to find these articles were sound and the studies they included represent most of the eligible studies worldwide that had collected information on oral contraceptive use and ovarian cancer.
- The authors caution that there is always a possibility that some studies may have been missed and that despite extensive efforts to identify studies with unpublished results, they cannot guarantee that they found them all.
- In addition, they say that some studies are still collecting data and could not contribute to the collaboration. However, these studies would have increased the number of cases by only about another 3 per cent, and are therefore unlikely to effect the overall result.
Observational studies such as this cannot identify exactly how the Pill may be causing the reduction observed, however it is likely that because the Pill works by suppressing ovulation, this may in part explain the beneficial effect and strengthens the confidence that this is a real effect.
Sir Muir Gray adds...
I don’t think there will be stronger evidence than this; these people are serious scientists. Good news for the pill and for pill takers.