Cancer Research UK – Science blog

You may have heard about a new study into contraception and cancer recently. New information about cancer risks can be hard to apply to everyday life, and headlines about this paper are highlighting some alarming-looking numbers. If you or someone you’re close to uses hormonal contraception, there’s a chance you’re asking what the latest findings actually mean. Let’s break this research down and see.

First, we should say a bit about what hormonal contraception is. Unlike barrier contraception, such as condoms, hormonal contraception releases hormones into a woman’s body to prevent her becoming pregnant. There are two main types:

• Combined contraception, which releases two hormones called oestrogen and progestogen.

• Progestogen-only contraception, which, as the name suggests, only releases progestogen.

Combined contraception usually comes as a pill or a patch. Progestogen-only contraception can be a pill, implant, injection, or intra-uterine device (IUD).

What did we already know about contraception and cancer?

Previous research has already established that the combined pill increases the risk of breast cancer. This increased risk is small and only lasts while someone is taking the pill. After they stop taking it, the increase in risk slowly disappears.

The combined pill also decreases the risk of womb and ovarian cancer. So, for most women, the benefits of using the combined pill outweigh the risks. Still, health guidelines encourage health professionals to inform their patients about the small difference it can make to their risk of breast cancer.

Up till now, the evidence on other types of contraception, such as the progestogen-only pill, and cancer has not been as strong as the evidence on the combined pill. This is partly because there weren’t as many people using other forms of contraception, which meant researchers couldn’t put together big enough studies.

However, since 2010 the number of people using the combined pill has gone down, and the number of people using the progestogen-only pill has gone up. By 2020, they were being used by almost equal numbers of people. Now that there are more people using progestogen-only contraceptives, researchers can more accurately study the population to see how they affect cancer risk.

What did this new study show?

There are two parts to this new study into contraception and breast cancer, which we helped fund. First, the authors compared the health records of nearly 10,000 people who had breast cancer to the records of about 18,000 people who weren’t affected by the disease. They looked at how many people in each group used contraception, as well as what types of contraception they used, to see how it impacted their risk.

After accounting for BMI and alcohol – two factors that we already know increase the risk of breast cancer – the researchers found that the progestogen-only pill, injection, and IUD have a similar effect on the risk of breast cancer as the combined pill. The results for the progestogen-releasing implant were too inconsistent to draw conclusions from, and the study didn’t investigate the effects of the combined patch.

However, the authors also realised that many women who have used hormonal contraception may have tried more than one type. It’s not unusual for women to use a few different types while they find the one that works best for them. This means the first analysis doesn’t provide enough evidence for us to be sure which type of contraception was having an effect on breast cancer, or whether the risk was actually linked to people using a combination of different types.

So, to round out part one, the researchers re-did their analysis. This time they limited it to women who had only ever taken one type of hormonal contraception. This meant the sample size was smaller, so the results are less reliable. Even so, it gives us a clearer idea of how specific types of contraception might affect people’s risk levels.

These results still show an increased risk of breast cancer from using the combined pill, progestogen-only pill and IUD. However, as with the implant in the first analysis, the second set of results is too inconsistent for us to draw conclusions about the injection.

Importantly, this follow-up analysis also showed that, similarly to the combined pill, the increase in breast cancer risk from using the progestogen-only pill goes away once someone stops using it.

The second part of the study was a meta-analysis. This means the researchers combined a range of smaller past studies that weren’t reliable enough by themselves into one larger, more reliable study. That gave more evidence that progestogen-only contraceptives increased the risk of breast cancer.

What does this mean for you?

We already knew that the combined pill has a small, temporary effect on the risk of breast cancer – and that this risk is outweighed by its benefits. This new research suggests other forms of contraception have a similar risk that also goes away after people stop using them.

It’s also important to remember that women who use contraception are usually below the age of 50. This means their risk of breast cancer is low to begin with, so any increase will have a relatively small effect.

As such, the researchers estimated that for every 100,000 women who use oral contraception from the age of 16 to 20, there will be an extra 8 cases of breast cancer (this is an increase in incidence from 0.084% to 0.093%). And for every 100,000 women who use it between the ages of 35 and 39, they estimate there will be around 265 extra cases (an increase in incidence from 2.0% to 2.2%).

So, the take-home message around cancer and contraception has not changed – for most women, the benefits of using contraception still outweigh the risks. The information from this new study means most women can continue using their contraception with a new level of understanding about their choices.

To put this into context, it’s estimated that in the UK, around 400 breast cancer cases are caused by the pill every year. Around 4,400 are caused by alcohol. This difference is partly because drinking alcohol is much more common than taking the pill.

If you’re worried about using contraception, speak to your doctor. The best way to lower your risk of cancer is to stop smoking, eat a healthy, balanced diet, be physically active and keep a healthy weight.

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