The pill provides 'lifelong protection against some cancers'
"The pill can protect women from cancer for 30 years," is the front page headline in the Daily Mirror.
The paper reports on a landmark study that followed more than 46,000 women in the UK for up to 44 years.
The study found women who'd used the combined oral contraceptive pill – commonly known as "the pill" – were less likely to get bowel (colorectal) cancer, womb (endometrial) cancer and ovarian cancer many years after they'd stopped taking the medicine.
Although women had a higher risk of breast cancer and ovarian cancer while taking the pill, the researchers say this raised risk "appeared to be lost" within about five years of having stopped taking it.
Researchers concluded the overall effect of taking the contraceptive pill was "neutral" when taking into account the balance of higher risks and lower risks.
But there are a number of uncertainties to consider with this research.
The study began in 1969, only a few years after the pill was first made available in the UK in 1961. Doses of oestrogen and progestogen were higher than many of the pills available now.
And the women in the study used the pill for an average of 3.5 years, which may be different from how women use the pill today.
Because of the type of study, we don't know for certain whether the change in cancer risk is because of the effects of the pill or whether other health or lifestyle factors had an influence.
Still, women who took the pill can be reassured it's unlikely to have had a long-term effect on their cancer risk.
Where did the story come from?
The study was carried out by researchers from the University of Aberdeen, and was funded by the Royal College of General Practitioners, the Medical Research Council, Imperial Cancer Research Fund (now part of Cancer Research UK), the British Heart Foundation, and several pharmaceutical companies that manufacture oral contraceptive pills (Schering, Wyeth Ayerst, Ortho Cilag and Searle).
It was published in the peer-reviewed American Journal of Obstetrics and Gynaecology.
The study was widely covered in the UK media, and the reporting was broadly accurate.
However, several headlines and stories failed to warn readers this type of study can't prove the pill was the reason for lower rates of certain cancers among women who took it.
For example, weight and alcohol are linked to cancer risk, but weren't measured in the study. We don't know whether women who took the pill were more or less likely to drink alcohol or be overweight.
What kind of research was this?
This long-term prospective cohort study aimed to look at whether the use of the combined oral contraceptive pill has an effect on long-term cancer risks.
One concern related to use of the pill is that as it uses hormones, it could have an effect on the risk of developing certain cancers known to be related to hormones, such as breast and ovarian cancers.
Observational studies like this are good ways to track patterns of risk, especially over the long term.
But they can't prove that one factor (in this case, taking the contraceptive pill) directly causes another (cancer or protection against cancer).
What did the research involve?
The study involved women taking part in the UK Royal College of General Practitioners' Oral Contraception Study in 1968-69.
Researchers followed-up 23,000 women who were using combined oral contraceptive pills at the start of the study, and 23,000 who hadn't used them at that point.
The women's GPs were required to fill in reports every six months on their hormone use, pregnancies, illnesses or deaths. This continued until 1996.
In the 1970s, three-quarters of the women were "flagged" in cancer registries so the study would be notified about any subsequent cancer diagnosis. This information provided the post-1996 data.
The study results have been reported at various times since it began. This paper looks at the longest period of follow-up.
Researchers had already seen a reduction in rates of ovarian, endometrial and colorectal cancers, and wanted to see if these reductions persisted into older age.
They also wanted to see if using the pill during childbearing years could produce new cancer risks in later life, and look at the overall balance of cancer risk among older women who used the pill in the past.
The researchers used the data to calculate the incidence rate of cancers in women who'd ever or never taken the pill.
They presented the results as the difference between the two incident rates (number of cancer cases per 100,000 women per year) and the percentage of cancers that might have been attributable to taking or not taking the pill.
Researchers adjusted their figures to account for the women's age, whether they smoked at the start of the study, whether they had children, and their social class.
What were the basic results?
There was very little difference in the chances of getting cancer overall. There were 542.44 cancers per 100,000 women per year among those who took the pill, and 566.09 among those who didn't take the pill.
This overall difference is small enough to be down to chance – in other words, it's not statistically significant.
- ovarian cancer risk was 22.1 per 100,000 women per year for pill users and 33.27 for non-pill users – women who took the pill were 33% less likely to get ovarian cancer (the difference between the incident rate for the two groups in the study [incident rate ratio, or IRR] 0.67, 99% confidence interval [CI] 0.5 to 0.89)
- cancer of the womb lining (endometrial cancer) risk was 19.42 per 100,000 women per year for pill users and 29.56 for non-pill users – women who took the pill were 34% less likely to get ovarian cancer (IRR 0.66, 99% CI 0.48 to 0.89)
- bowel (colorectal) cancer risk was 47.85 per 100,000 per year for pill users and 59.16 for non-pill users – women who took the pill were 19% less likely to get colorectal cancer (IRR 0.81, 99% CI 0.66 to 0.99)
Women who took the pill were 48% more likely to get breast cancer while they were taking it and for five years afterwards, but the difference in risk disappeared five years after stopping treatment.
The researchers concluded there was no overall increase in risk when the whole period of the study was considered.
How did the researchers interpret the results?
The researchers said: "Our results suggest that users of oral contraceptives are protected from colorectal, endometrial and ovarian cancer for many years after stopping, perhaps for more than 35 years for colorectal and ovarian cancer.
"Most women who choose to use oral contraceptives do not expose themselves to long-term cancer harms."
The research is in line with other studies that have reported on cancer risk and the pill.
This study had the advantage of being both very large and having the longest follow-up period of any study of the effects of the pill on cancer.
But we shouldn't lose sight of this study's limitations.
It's not possible to say that taking the pill prevented women from getting certain cancers. It may be the case, but other confounding factors could be involved.
The researchers took account of some basic factors that affect cancer risk, but not others like diet, physical exercise, weight and alcohol use.
Many of the women in the study were lost to follow-up, mainly because they moved away or otherwise lost touch with their GP before they could be flagged for the cancer registries. But it's unlikely that this would affect pill users or non-pill users differently.
Hormonal contraception – and women's use of it – has changed a lot since the study began in 1968.
The composition and dose of hormones used in the combined oral contraceptive pill has changed considerably since they were first introduced.
This means we can't be sure the effects of the contraceptive pills of almost 50 years ago would be the same as those of today's pills.
This study didn't assess other types of combined contraception, such as the contraceptive patch, and didn't look at progestogen-only contraceptives, such as the "mini-pill", implants, injections and intrauterine systems. This means the results can't be applied to "hormonal contraception" in general.
These uncertainties aside, this study provides reassuring news for older women, who may wonder whether the oral contraception they used in their younger years has increased the risk of getting cancer.
There's no evidence from this study of an increase in cancer risk – and the reverse may be true for certain types of cancer.
If you're looking for alternatives to hormonal contraception, condoms are 98% effective when used correctly.
Read more about contraception.
Links To The Headlines
The Pill can protect women from cancer for 30 years as breakthrough research gives new hope. Daily Mirror, March 22 2017
Contraceptive pill protects women against cancer for 35 years, major study suggests. The Daily Telegraph, March 22 2017
Contraceptive pill 'can protect against some cancer types for 30 years'. The Independent, March 22 2017
Using the pill can protect women from certain cancers 'for up to 30 years'. The Guardian, March 22 2017
Women who take the contraceptive pill are 'a third less likely' to develop these cancers. The Sun, March 22 2017
Oral contraceptive pill can protect against cancers for 30 years. The Times, March 22 2017 (subscription required)
Links To Science
Iversen L, Sivasubramaniam S, Lee AJ, et al. Lifetime cancer risk and combined oral contraceptives: the Royal College of General Practitioners' Oral Contraception Study. American Journal of Obstetrics and Gynecology. Published online February 8 2017