It is a common thing to hear among women discussing contraception, and in the press:

A 45 per cent increase in the disease was found among women who had been taking the second generation Pill for three years or more. A ten per cent increase was found in those taking it for less than three years. The findings follow a study of 100,000 women.

But is there a link?

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    This question really depends on which pill and which cancer. – Monkey Tuesday May 11 '11 at 21:56
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    Here is the 2003 study being reported on. (The date of the Daily Mail article is not made clear on the web-site, which I find surprising.) – Oddthinking Oct 17 '11 at 1:00
  • One thing that is rarely considered when scare comments like this are made: even if there is an increased risk of cancer, that extra risk is far smaller than the extra risk from pregnancy. – hdhondt Apr 30 '13 at 4:47
  • Actually for some cancers, there seems to be a link between less (or at least at later age) pregrancies (or rather more periods) to more cancer (I recall a study in Japan, don't have the referrence on me though). – Ofir Apr 30 '13 at 7:44

What I did was enter "cancer contraceptive meta" into the Google Scholar search engine. There are various disagreeing results. One of the more recent meta studies concludes:

Background: Previous studies convincingly showed an increase in risk of breast cancer associated with current or recent use of oral contraceptives from the 1960s to 1980s. The relation of contemporary oral contraceptive formulations to breast cancer risk is less clear.

Methods: We assessed lifetime oral contraceptive use and the specific formulations used among 116,608 female nurses ages 25 to 42 years at enrollment in 1989, and subsequently updated this information every 2 years. We related this information to risk of breast cancer up to June 1, 2001.

Results: During 1,246,967 person-years of follow-up, 1,344 cases of invasive breast cancer were diagnosed. Past use of any oral contraceptive was not related to breast cancer risk [multivariate relative risk (RR), 1.12; 95% confidence interval 0.95-1.33]. Current use of any oral contraceptive was related to a marginally significant higher risk (multivariate RR, 1.33; 95% CI, 1.03-1.73). One specific formulation substantially accounted for the excess risk: the RR for current use of triphasic preparations with levonorgestrel as the progestin was 3.05 (95% CI, 2.00-4.66; P < 0.0001).

Conclusions: Current use of oral contraceptives carries an excess risk of breast cancer. Levonorgestrel used in triphasic preparations may account for much of this elevation in risk.

Impact: Different oral contraceptive formulations might convey different risks of breast cancer; ongoing monitoring of these associations is necessary as oral contraceptive formulations change. Cancer Epidemiol Biomarkers Prev; 19(10); 2496–502. ©2010 AACR.

So yeah, since there is not the one pill, results differ among these. The risk is only marginally significant, which means that if one throws levonorgestrel out of the mix, the risk will probably not be significant any more.

Reference: http://cebp.aacrjournals.org/content/19/10/2496.short

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    I'm interested how the authors, or perhaps even you, explain the results with respect to results of the 2003 study. – Oddthinking Oct 17 '11 at 1:02

A large study from the UK published in the BMJ in 2010 suggests that pill users are not at extra risk compared to non-users. This study was designed to explore mortality from many causes not just cancer and its scale and longevity would tend to suggest it is a robust analysis. the design:

Prospective cohort study started in 1968 with mortality data supplied by participating general practitioners, National Health Service central registries, or both.

The results (my highlights for cancer related mortality):

Compared with never users, ever users of oral contraception had a significantly lower rate of death from any cause (adjusted relative risk 0.88, 95% confidence interval 0.82 to 0.93). They also had significantly lower rates of death from all cancers; large bowel/rectum, uterine body, and ovarian cancer; main gynaecological cancers combined; all circulatory disease; ischaemic heart disease; and all other diseases. They had higher rates of violent deaths. No association between overall mortality and duration of oral contraceptive use was observed, although some disease specific relations were apparent.

The study concludes:

Oral contraception was not associated with an increased long term risk of death in this UK cohort and may even produce a net benefit (absolute risk reduction among ever users 52 per 100 000 woman years)

The balance of risks and benefits, however, may vary around the world, depending on patterns of oral contraception usage and the prevalence of different diseases

The caveat at the end is partially a result of the cohort being relatively healthy which leaves the possibility that certain pre-existing risks might interact with the pill in ways that increase mortality.

Overall, though, it doesn't look like the pill increases cancer rates in a healthy population.

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    Similar to the other answer: I'm interested how the authors, or perhaps even you, explain the results with respect to results of the 2003 study. – Oddthinking Oct 17 '11 at 1:03
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    @Oddthinking My initial guess is sample bias. The 2003 study worked backwards from breast cancer diagnoses and asked women about past pill use. Studies like this have a habit of introducing biases that are very very difficult to correct. One in particular might be that the retrospective sample biases the cohort to include people susceptible to breast cancer whereas the prospective studies usually exclude certain less healthy subgroups. Perhaps a small number of people are either particularly sensitive or engage in riskier behaviour. – matt_black Oct 17 '11 at 18:23

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