It is estimated that as many as 140 million women worldwide take the oral contraceptive pill. It is a simple, effective way for a female to control her own fertility and has also been utilised as a treatment for heavy periods, acne and endometriosis. However, safety concerns over its usage persist, and one of the major issues is a suspected link between the pill and cancer.
So, what does the science say? Does taking the pill increase your risk of developing cancer, or can it actually serve a protective role?
The pill as an anti-cancer agent
The pill reduces the risk of ovarian, endometrial and colorectal cancer.
- Ovarian cancer. Ever-users of the pill are significantly less likely to develop ovarian cancer than never-users. In fact, studies have suggested that for every 5 years of pill use there is a 20% reduction in ovarian cancer risk. Furthermore, these protective effects are maintained for at least 30 years after discontinuation of pill use.
- Endometrial cancer. The pill exerts a substantial protective effect against the development of endometrial cancer. The risk is thought to be reduced by between 30 and 50%, depending on the duration of use. Protection lasts for at least 20 years after cessation of treatment.
- Colorectal cancer. Ever-users of the pill are approximately 15% less likely to develop colorectal cancer than those that have never taken it. Whether increased duration of use has a beneficial effect is difficult to know as the quality of data from these studies is poor.
The pill’s carcinogenic effects
Oral contraceptive use is a risk factor for breast and cervical cancer.
- Breast cancer. The association between breast cancer and the oral contraceptive pill is small, but significant. Studies suggest that those women who are currently on the pill are 20% to 24% more likely to receive a breast cancer diagnosis. The increased risk is lost once the pill is discontinued and, after ten years, prior pill users are at no greater risk than never users of getting breast cancer. The risk also falls significantly once a woman goes through the menopause.
Pill use is associated with clinically challenging types of breast cancer, including the triple negative form, which usually has a worse prognosis and higher mortality rate. The exact mechanisms linking the two are unclear, although many breast cancers have a hormonal component. It is thought that increased lifetime exposure to oestrogens increases the risk of breast cancer, primarily because the hormone promotes or initiates tumour growth.
Studies have failed to find an elevated risk of breast cancer in pill users with a family history of the disease. However, the data may be skewed by the fact that these women are less likely to use the pill due to their already increased susceptibility.
The established link between oestrogen and breast cancer may lead you to wonder whether using the progestin-only ‘mini pill’ would be a safer option. There have been very few studies on this form of contraception, probably because it is not as widely used as the combined pill (which contains oestrogen and progestin). The work that has been performed has suggested that women who take the mini pill still have a higher risk of breast cancer than those who have never used oral contraceptives, perhaps by as much as 21%. The link between progestins and breast cancer is poorly understood and likely to be complex. However, it is validated by studies on postmenopausal women who take hormone replacement therapy. Those on combined oestrogen plus progestin therapy have a higher breast cancer risk than those who take just oestrogen. Thus, the mini pill should no longer be considered a safer contraceptive option for those with an elevated risk of developing breast cancer.
- Cervical cancer. Taking the pill for five or more years is associated with a significantly increased risk of developing cervical cancer. The duration of use increases the risk, so that women who take the pill for 5 years have a 10% increased risk, but those who take it for longer, are more than 60% more likely to be diagnosed. Ten years after stopping the pill there is no increased risk of developing cervical cancer.
One important thing to note is that as a risk, oral contraceptives will always be cofactors that interact with high risk human papillomavirus (HPV) strains to induce cervical carcinogenesis. This means that oral contraceptives in isolation are not a risk factor in women who are HPV negative; however, for those who are HPV positive, the pill can exacerbate the risk. Steroid hormone receptors (mainly progesterone) are found in cervical tissue and are thought to enhance the expression of high risk HPV, contributing to cancerous changes in the cervix.
Conclusion: the pill and cancer
It is very difficult to state conclusively whether the pill should be used or avoided based on its associations with cancer. The net effect is likely to be positive, with one long-term, UK-based study finding that taking the pill resulted in a 12% reduction in overall cancer risk.
It is also challenging to assess the absolute risk posed by oral contraceptive use. Cancer can have a long latency period, meaning the time between exposure to a particular risk factor and cancer diagnosis can span many years. Most women who develop cancer will have been exposed to multiple risk factors during their lifetimes, including parity (the number of times she has been pregnant and carried the baby to term), obesity and whether or not she has breastfed (breastfeeding can exert protective effects). Ascertaining how much of a role each of these factors plays in a later cancer diagnosis is likely to be extremely difficult.
One final thing to consider is that the pill has changed in formulation over the decades since it was first utilised as a type of contraception. The specific synthetic hormones in use have changed, as has their concentration. Today, a triphasic pill is commonly used, whereby the hormone concentration changes across the month. This is designed to more closely mimic the normal ovulatory cycle. These different formulations will have differing risks and benefits. The consequence of this is that women who were prescribed the pill in its infancy, in the 1950s and 1960s should certainly not be compared to those taking it today in the 21st Century. Perhaps the advent of the mini pill came with initial optimism that the risks associated with the combined pill would be alleviated by removing the oestrogen component. Unfortunately, it appears that women taking this form of contraceptive have a comparable cancer risk to those taking the more commonly prescribed combined oral contraceptive pill.
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