The Contraceptive Pill And Strokes
Putting new research into the pill in context
One of the first things I ever wrote about when I started my scientific blog was the contraceptive pill. While I have no great insight into the experience of hormonal contraception - I’m a man lacking the requisite organs - I’ve always thought that the hate that contraception gets in the media is far out of proportion of the scientific evidence.
I’ve never been able to tell if it’s because people are weird about periods, because of the whole societal hatred of sex, or just because contraception goes against a certain religious mindset that requires women to be eternally pregnant. Maybe a combination of all three?
Regardless, the recent news seems to fit the trend. Headlines are decrying the medication as dangerous and saying that it could double or even quadruple your risk of strokes and heart attacks. For anyone who has or is using the pill to prevent pregnancies - or for one of the many other things that the pill can be used for - it must be quite worrying.
Fortunately, the science isn’t nearly as concerning as the headlines make out. Let’s look at the data.
Pill Publications
The new study that’s being reported in all of these media stories is a very large cohort study looking at women in Denmark. Scandinavian countries are the best in the world at collecting useful data into massive databases that allow for detailed observational research. Because of this, the researchers here were able to include every Danish woman aged 15-49 from 1996 to 2021. This gave them a cohort of just over 2 million people.
The researchers then compared the risk of stroke and heart attack for a variety of options including oral contraceptive pills, hormonal implants, and hormonal IUDs against the risk for people who used none of these contraceptives. The researchers found that there was an increased risk of stroke and heart attack for all forms of combined oral contraceptives - the dual-hormone pill, patch, or vaginal ring - where there was enough data to run the analysis.
Single-hormone - usually called progestin-only - oral contraceptives also showed an increased risk. However, the hormonal IUD, implant, and injections did not show any of the same issues, although again injection use was very low and so there wasn’t much data to rely on for that analysis.
Some of these ratios were fairly high. People in the study using a combined vaginal ring for contraception had 3.8x higher rates of heart attacks than those who used no hormonal contraceptives.
But before you throw away your pills, there’s some important context here.
If you present just the risk ratios, the data looks very scary. Doubling or tripling your risk of a heart attack sounds like a very big thing! But these events are incredibly rare for young women*. 0.000002% is double 0.000001%, but neither are risks that we necessarily care about in our daily lives.
In this study, about 2 in every 10,000 people who used no hormonal contraceptives had a stroke each year. This went up to about 4 in every 10,000 each year for people who used the highest-risk combined pill. For heart attacks, the baseline risk was 1 in 10,000, which increased to around 2 in 10,000 for the higher-risk contraceptive options.
These are not meaningful risks for the average person. They’re important at the population level, because even an 0.02% risk happens quite a bit if you’re looking at more than a million people, but they’re rare enough that they don’t really impact individual choices.
There’s also some important additional context when it comes to contraceptive medications. For one thing, they prevent pregnancy, which itself causes strokes and heart attacks at a similar or higher rate than any of these medications. This study also had all of the usual biases that observational research had, meaning it’s not clear that the association between contraceptives and heart attacks/strokes was causal. A 2023 study using almost identical methodology but looking at a UK cohort found almost exactly the opposite result.
So the contraceptives may still prevent more events than they cause. They also might not really be causing the issues seen in the study. The researchers had access to an amazingly robust dataset, but their statistical model was surprisingly simple and they didn’t take much time to really check if the heart attacks and strokes were being caused by the pills or not. It’s possible that the risk may be overstated by a fair bit in these results.
Bottom Line
This is a very useful piece of research if you, like me, are a population scientist looking at risks for millions of people. But for the average human being there’s not much useful data here for personal decision-making. Most people already know that contraceptive pills may cause an increased risk of strokes, and are also aware that it’s a very rare complication that is mostly concerning to people who are already at high risk of heart issues.
It’s also worth noting that the hormonal IUD was the third most popular contraceptive choice in this study - coming after none, and the combined pill - and it was the safest option. If you’re very worried about the possibility of an increased risk of heart problems from the pill, there’s at least one very popular choice which doesn’t appear to have any of the same heart health concerns.
That being said, I don’t think these results are particularly meaningful to you, the person who’s actually taking the pill. There does appear to be a possibly increased risk of strokes and heart attacks associated with using the medication, but it’s very rare.
*Note: Yes, in this context “young” includes all women under 65. However you personally define it, 50 is still very young for cancer from a population perspective.