Wednesday September 12 2007
Does the pill reduce cancer risk?
Using the contraceptive pill has been shown to reduce the risk of women developing cancer, reported The Times and other newspapers. The findings, they say, are the result of a 36-year study that showed that overall risk for developing cancer (including bowel, uterine and ovarian cancer) “was up to 12% lower for women who took the pill for less than eight years”. However, this reduced risk only seems to be associated with shorter-term use, as the risk of cancer increased if the pill was taken for more than eight years. The Daily Mail concluded: “The pill protects against cancer as well as pregnancy.”
The original research is a large and reliable long-term study. However, interpretation of the findings requires some care, and it may be premature at this stage to conclude that taking the pill protects against cancer.
Where did the story come from?
Professor Philip Hannaford and colleagues of the University of Aberdeen carried out this research. Funding was provided by Royal College of General Practitioners, Medical Research Council, Imperial Cancer Research Fund, British Heart Foundation, Schering AG, Schering Health Care, Wyeth Ayerst International, Ortho Cilag, and Searle. The study was published in the peer-reviewed British Medical Journal.
What kind of scientific study was this?
This was a cohort study designed by the Royal College of General Practitioners to investigate the risks and benefits from use of the oral contraceptive pill.
The study began in May 1968, when about 23,000 women who had never used the pill and 23,000 women currently using the pill were recruited through GPs across the UK. Initial information on social status, lifestyle and past medical history was collected at the start of the study, then every six months updated information was collected from the GPs about hormones prescribed, any pregnancies, or any medical problems. Follow-up continued with the GP unless the participant moved away, changed GPs or the GP left the study.
During the 1970s, the NHS central registry was used to identify three-quarters of the original study participants, and subsequent information on cancers and death rates was recorded. GP follow-up continued until December 1996. Data was collected on cancer rates among those women whose lifetime contraceptive status was accurately known (this group was known as the ‘main dataset’) until December 2004. Cancer rates were calculated and compared for those who never used the pill and those who had used it, adjusting for factors such as age, smoking, number of children, and HRT use.
What were the results of the study?
The researchers found that, overall, in the ‘main dataset’ group, the risk for any type of cancer was reduced by 12% among women who had ever taken the pill compared with women who had never used the pill. In the smaller set of women who received full GP follow-up, the reduction in risk of any cancer was smaller and was not statistically significant (i.e. there is no certainty that these results show a true reduction in cancer risk). Significant reductions in risk were found individually for cancers of the large intestine, uterus, and ovary. A slight increased risk was found among pill users for the risk of cervical cancer but this had no statistical significance.
The average length of contraceptive pill use was 44 months. The researchers found that use for eight years or longer was associated with an overall increase in the rate of any type of cancer compared with the group who had never used it, particularly with cancer of the cervix and of the central nervous system or pituitary gland in the brain. Conversely, the risk of ovarian cancer was significantly decreased in these women.
What interpretations did the researchers draw from these results?
The researchers conclude that pill use was not associated with an increased overall risk of cancer in the UK women they studied, and that it may in fact be protective against cancer. They acknowledge, however, that the pattern of cancer risks and benefits may vary across countries depending on types and rates of pill use and occurrence of different cancer types.
What does the NHS Knowledge Service make of this study?
This is a very large and reliable UK study that adds further evidence to already well-established theories that the contraceptive pill may reduce the risk of uterine and ovarian cancers, while slightly increasing the risk of cervical cancer. However, there are several points to be aware of when interpreting the findings of this study.
- There was a fairly high loss to follow-up for the ‘main dataset’ of results, with only 67% of the original women completing the study. Assumptions also had to be made for these women about their contraceptive use after GP follow-up ended in 1996; for example it was assumed that women aged over the age of 38 in 1996 who had not previously used the contraceptive pill would not start to use it. This may have introduced some inaccuracies. The GP dataset alone showed much smaller and non-significant reductions in overall cancer rates.
- The data relied on the NHS central registry to have complete and accurate records of all cases of cancer, which may have led to some cancers being missed.
- Although certain risk factors could be adjusted for, such as age, smoking, HRT use and social status, others may not have been considered. For example, a lower rate of large bowel cancer was found among people who had used the pill; however, no risk factors for bowel cancer, such as family history, appear to have been taken into account when looking at either group. Therefore it cannot be said for certain that the use of the pill, rather than other factors, is reducing the risk of bowel cancer in these women.
- In reporting the potential protective cancer benefits of taking the pill, both the study and the newspapers have not considered other risks associated with contraceptive pill use. For instance, women taking the combined pill are known to be at a slightly increased risk of developing the serious medical complication of deep vein thrombosis.
- It is also important to realise that types of oral contraceptives used (i.e. concentration of hormones in the pills), and patterns of use among women today are different from when the study began 40 years ago; therefore the findings from this study may not necessarily be applicable to women now. For example, today, starting the contraceptive pill at a younger age and using it for longer durations is likely to be far more common than it once was.
Sir Muir Gray adds...
All drugs can cause harm as well as benefit. When a drug, like the oral contraceptive, is taken by healthy people, it is important that the risks should be as low as possible. When a study shows that a risk reduction has been found, it is unlikely that anyone will take the medication for this reason. They will take continue to take the pill for its primary purpose, to prevent pregnancy, and the study allows them to do so with better information about possible risks.
As always, readers should be cautious about any statement regarding ‘cancer’. There are many different cancers and, as this report shows, responses differ between one type of cancer and another.