Contraceptive pill 'cuts womb cancer risk'

“The pill cuts womb cancer cases by 200,000,” the Sky News website reports, in an uncommon example of a credible headline figure. However, it should be noted that it refers to the amount of cases prevented over 10 years.

The news follows a reliable review that found the longer women took the pill, the lower their risk of womb cancer. As demonstrated in the Sky News headline, the risk reductions were quite large – using the pill for about 10 to 15 years halved the risk of womb cancer (sometimes known as uterine or endometrial cancer). This effect lasted up to 30 years after oral contraception had stopped.

Researchers pooled the results of 36 studies including more than 140,000 women. It aimed to compare past use of combined oral contraceptives – aka the pill – in women with and without womb cancer. The results are not relevant to the progesterone-only “mini-pill”.

The results showed that protection from womb cancer was the same for women taking the pill during the 1960s, 70s and 80s, even though earlier pills contained much higher oestrogen levels.

This finding isn’t new – the pill was already recognised to reduce risk of womb cancer, but this study has pooled the evidence to show just how big the link may be.

Cancer of the womb is relatively common and abnormal vaginal bleeding is the most common symptom.

There are a range of contraceptives on offer, not just hormonal pills. Each has its own pros and cons, and if you’re using contraception you should probably consider which might be the best contraceptive method for you.

Where did the story come from?

The study was carried out by a large group of researchers called the Collaborative Group on Epidemiological Studies on Endometrial Cancer and was funded by the Medical Research Council and Cancer Research UK.

The study was published in the peer-reviewed medical journal The Lancet Oncology.

The Guardian, Sky News and Mail Online all reported the study facts accurately. All reported that every five years of taking the pill reduced the chance of womb cancer by a quarter, and that this probably prevented about 200,000 cancer cases in the past decade.

The Guardian headline said: “Regularly taking the pill ‘helps prevent two forms of cancer’ decades after use”. This refers to the current study on womb cancer as well as a study published in 2008 that found similar effects of the pill on the risk of ovarian cancer.

What kind of research was this?

This was a systematic review and meta-analysis that aimed to investigate the link between oral contraceptives and womb cancer.

Cancer of the womb (uterus) is a common cancer. Abnormal vaginal bleeding is the most common symptom of womb cancer. Read more about womb cancer.

The combined oral contraceptive pill – commonly called the pill – is already known to reduce the risk of endometrial cancer, but it is unclear how long this effect lasts after contraception is stopped, or whether it is modified by other factors like smoking or body weight.

A meta-analysis is a great way to investigate this issue. It pooled the results of many studies to find an overall result. By pooling lots of data, the reliability of the end result increases. This method relies on finding different studies investigating a similar issue in a similar way; otherwise, pooling the results isn’t a good idea.

What did the research involve?

The researchers pooled data from 36 studies that comprised a total of 27,276 women with endometrial cancer (cases) and 115,743 without (controls). They were looking for any statistically significant links between oral contraceptive use and cancer cases up to 30 years later.

The research team searched medical databases to identify studies measuring oral contraception use and endometrial cancer. This included contacting study authors for unpublished data.

Cases were defined as women with invasive cancer of any part of the uterus who were without previous cancer. The controls were women without previous cancer who had an intact uterus.

Most studies reported whether or not women had ever used hormonal contraceptives and most also provided information about the total duration of use and age or calendar year at first and last use.

Only 13 studies collected information on the type of hormonal contraceptives. Women from the remaining 23 studies were assumed to be using combined oral contraceptives, containing both oestrogen and progestogen, because more than 95% of hormonal contraceptive users included in studies with such information reported using combined preparations.

The analysis took into account any of the women’s factors known to affect cancer risk in an attempt to isolate the effect of oral contraceptives. These included their:

• age
• number of births
• body mass index
• smoking habits
• use of hormone replacement therapy (HRT)

The researchers weren’t able to analyse women with endometrial cancer who had used exclusively progestogen-only oral contraceptives (sometimes called the “mini pill”), or sequential oral contraceptives (where separate pills contain oestrogen only or combined with progestogen 41 cases). This is because there were too few of these cases to do thorough analyses.

What were the basic results?

The average age of women with endometrial cancer (cases) in the study was 63. The researchers found 35% of cases reported using oral contraception in the past (for an average of three years), and 39% of controls had taken the pill (average 4.4 years’ use).

The longer the women used oral contraception, the more it reduced the risk of endometrial cancer. For example, for every five years women used contraception, the risk of endometrial cancer reduced by 24% (risk ratio (RR) 0.76, 95% confidence interval (CI) 0.73 to 0.78). This means that about 10 to 15 years of using the pill should halved the risk of endometrial cancer.

This reduction in risk persisted for more than 30 years after oral contraceptive use had stopped, with no apparent difference in risk between pills taken during the 1960s, 1970s, and 1980s, despite higher oestrogen doses in pills used in the early years.

There were some interesting subtleties in the results; particularly, the reduction in risk associated with ever having used oral contraceptives differed according to the type of cancer. There was a reduced risk for carcinomas – cancers of the lining of the uterus or womb (RR 0·69, 95% CI 0·66 to 0·71), but no significant effect on risk of sarcomas – cancers affecting the muscle or supporting tissue around the womb (RR 0·83, 95% CI 0·67 to 1·04).

In high-income countries such as the UK, 10 years use of oral contraceptives was estimated to reduce the absolute risk of endometrial cancer arising before age 75 years from 2.3 per 1,000 women to 1.3 per 100 women.

How did the researchers interpret the results?

The researchers said: “Use of oral contraceptives confers long-term protection against endometrial cancer. These results suggest that, in developed countries, about 400,000 cases of endometrial cancer before the age of 75 years have been prevented over the past 50 years (1965-2014) by oral contraceptives, including 200,000 in the past decade (2005-14).”

Conclusion

This review found that the longer women had taken the combined oral contraceptive pill (the pill) the greater their reduction in risk of endometrial cancer. The risk reductions were quite large – use for about 10 to 15 years halved the risk – and lasted up to 30 years after oral contraption had stopped.

The protection did not seem to depend much on the dose of oestrogen in the contraceptive formulations or on personal characteristics of the women, such as how many children they had given birth to, their body mass index or whether they were menopausal.

The study was large and is likely to have included most of the studies on the topic. The analysis was also reliable, creating precise risk estimates over long periods of time. These points all increase our confidence in the findings.

No research is without limitations, and in this case the analysis was only as reliable as the studies included. For example, not all studies had complete and detailed information on oral contraceptive use for all women. However, if this had an effect on the result, it probably wasn’t large.

The researchers say that the pill used by women in the 1960s would generally have contained much higher doses of oestrogen than those of the 1980s. Despite this, they didn’t find any differences in the risk reductions between the years. They interpreted this to mean that: “the amount of oestrogen in the lower-dose pills is still sufficient to reduce the incidence of endometrial cancer, which is consistent with findings from two studies that have assessed individual dosages of the hormonal constituents”.

The pill is not without risks and is not suitable for all women. There is well-known risk of blood clots and some women may be at higher risk, such as smokers, those who are overweight or obese, and those with migraine or existing heart or vascular conditions. They have also been linked to increased risk of breast and cervical cancer.

Prof Valerie Beral, lead author of the study, explained the implications of the research to The Guardian: “There is an increase in cancer of the breast and cervix, but it is really quite small and they don’t persist.” The Guardian itself added: “Once a woman stops taking the pill, her increased chances of breast or cervical cancer quickly disappear.”

There are a range of hormonal and non-hormonal contraceptive methods on offer, not just oral pills, each with their own pros and cons. Find out which is best for you by following our contraception guide.

Links To The Headlines

Regularly taking the pill ‘helps prevent two forms of cancer’ decades after use. The Guardian. August 5 2015

The Pill Cuts Womb Cancer Cases By 200,000. Sky News. August 5 2015

The Pill could protect against womb cancer for five decades: Women who use drug for five years in their twenties can reduce chance of disease in their sixties and seventies by a quarter. Mail Online. August 5 2015

Links To Science

Collaborative Group on Epidemiological Studies on Endometrial Cancer. Endometrial cancer and oral contraceptives: an individual participant meta-analysis of 27 276 women with endometrial cancer from 36 epidemiological studies. The Lancet Oncology. Published August 4 2015