The pill and gliomas, is there cause for worry or is it too soon?

Hormonal contraceptives, also known as the pill, are among the most common type of birth control. They are highly effective with less than 1% of users experiencing a pregnancy per year. Beyond the ability to help with the prevention of pregnancy and family planning, the pill has been shown to be effective in helping control particularly painful cramping during monthly periods as well as aiding in more complex disorders like polycystic ovary disease. While the pill has become a staple for women looking to prevent unwanted pregnancies, there has been concerns raised in the past about cancer risks, and a new study has raised concerns about a specific form of cancer, gliomas or brain tumour.

The study, published in the British Journal of Clinical Pharmacology, looked into whether the use of hormonal contraceptives increased the risk of developing gliomas, the most common type of brain tumour. The researchers used a method known as a case-control study where they took a set of patients known to have glioma (317 patients) and compared how many of them used the pill to a set of control patients who didn’t have gliomas (2,126 patients). They found that patients who had ever used the pill for 5 years were more likely to have gliomas then those who didn’t. They also saw that this risk was associated with how long the person had been using the pill. The risk was greater for women using the progesterone only therapy than those using a combination therapy with estrogen or with estrogen alone. This is an alarming result so what does it mean?

You will note that I did not quote percentage increases in the risk here because without knowledge of the overall incidence rate of glioma it is essentially meaningless to the anyone. The incidence rate of gliomas is around 5 per 100,000 people or in other words you have about a 1% chance of developing glioma in your lifetime. That means that a 50% increase in risk (as found in this study for anyone who ever used the pill, regardless of type) means that there will be an additional 2-3 people per 100,000 that will develop glioma if they use the pill or a 1.5% chance. The researchers suggest that any analysis of risk versus benefit for hormonal contraceptives would still recommend the use of these contraceptives, primarily because more work needs to be done to determine what is occurring in these women.

This type of study, case-control, does not provide clear evidence that an exposure causes a disorder, only that the two are related. There are many other things to take into consideration including the smoking status, diet, exposure to radiation, occupation, prior history of cancer and genetic susceptibility of these women that may affect their chances of getting cancer. Also, this data is for a population in Denmark, the results may not be attributable to other populations around the world.  A more definitive study would need to be done to follow women who had taken contraceptives, see if they develop cancer, and compare them to those women who hadn’t taken the contraceptive. This is known as cohort study.

The data is far from conclusive. For one, men are more likely to get gliomas then women and we know men are not on hormonal contraception. Second, 50% of the women in the control group (so over 1000 people) were taking a contraceptive and had no gliomas. Clearly we haven’t figured out the whole story. The women who were unlucky enough to get gliomas while taking the pill likely had other confounding factors like genetic predispositions or environmental exposures that combined with the pill to produce cancer. Further is the fact the pill is protective against cancers of the ovary and endometrium (lining of uterus), which are much more common than gliomas, about 14 per 100,000 women for ovarian cancer and 16-20 per 100,000 women for endometrial cancers. Add to that the fact that a large meta study in 2013 found that the pill was associated with a lower risk of glioma development in over 4800 cases with glioma and over 14,000 controls across 11 studies.

Confusing, I know, but if you are worried about your risk, especially if you have a family history of cancers, talk to your doctor. They will be able to inform you of all the risks associated with the pill and can do regular monitoring to detect early changes that may signal problems. There are also other solutions for your needs if you decided the pill is too risky.

It is too early to call it quits on the pill, the water is muddy, but this research should be a call to action for other researchers to figure out what is truly going on. After all, science is not made up of one study but a collection of results over a number of studies that leads to a consensus result and clearly, we have no consensus yet.

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2 thoughts on “The pill and gliomas, is there cause for worry or is it too soon?

  1. I’m really glad you touched upon the statistics of this study. In medicine we learn to analyze the data statistics properly and make note of the validity of the study to make the best decisions for our patients. However it is truely easy to forget that the base incidence is one of the most important factors when looking at risk reduction. A relative risk of 50% looks impressive and frightening taken at face value when in reality the absolute risk is only 1.5 % more than baseline risk. A sneaky game played by statistics – it really goes to show you need to take a good look at the evidence instead of taking it at face value.

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