Blood-pressure pills appear to be linked to higher risks of cancer, according to a big review study making headlines. Should these results be at least a little worrisome to you if you’re on one of these medications? In my opinion, yes–it’s worth discussing with your doctor. (Please don’t stop taking your pills on your own.) This study has many of the earmarks of more trustworthy research. It’s a meta-study, which means it looked at the results from several previous studies, so that if some of the studies were troubled there’s still some hope the results from the better studies will partly compensate for those problems. The studies were randomized, controlled trials, which means the people in them were randomly assigned to either take the medication in question or not take it, which can eliminate a lot of the problems studies run into. (I’m oversimplifying, but that’s the basic idea.) And the studies were large, totaling some 60,000 patients, so there shouldn’t be as big a chance for statistical flukes as there would be in small studies. None of these characteristics guarantee reliable findings, but they can all help.
It’s worth noting that some articles describing this new study report that the study found that people on the drugs have a one percent higher risk of getting cancer. But other articles say the findings indicate risks go up on average about 10 percent, and for lung cancer a whopping 25 percent. The seeming contradiction here is due to the media’s ongoing, near-complete and deeply problematic failure to specify and make clear the difference between absolute risk and relative risk. When your absolute risk of contracting a disease goes up by, say, ten percent, it means that, for example, if your risk of the disease was 14 percent, it is now 24 percent (14 plus 10), a big difference. When your relative risk goes up by ten percent, then if it was 14 percent, it is now around 15 percent (14 plus ten percent of 14)–not such a big difference. So in this example, an increase in relative risk of about 10 percent translates to an increase in absolute risk of a mere 1 percent. Research institutions typically want the public to be impressed with the importance of their results, so they often emphasize to the press relative risk, which is a generally misleadingly high number. But in this case–perhaps because there is concern that people on blood-pressure pills will overreact and stop taking them–there seems to be more interest than usual in reporting the change in absolute risk, which tends to be a very low number. Assuming you don’t carry any special risk factors for lung cancer (such as smoking), you only have a tiny chance of getting it, and if you add on the apparent increased risk from blood-pressure pills, you now have a slightly less-tiny chance of getting lung cancer. Still, your chances of getting cancer of any type aren’t so tiny, and a one percent increased absolute risk is nothing to sneer at. A percentage here, a percentage there, and pretty soon you’re taking years off your life.
If you read about the study, you might note that even though the study found an increased risk for blood-pressure-pill takers of getting cancer, it didn’t find an increased risk of them dying from it. Unfortunately, you shouldn’t take this as reassuring. Most cancer studies fail to convincingly link increased risk factors to mortality, usually simply because it tends to take people many years to die of cancer (if they do die from it, that is), and studies usually don’t go on long enough to follow people to their graves. (That’s another reason, by the way, that researchers love running studies on mice instead of people–they’re conveniently short-lived. We humans can be annoyingly enduring, from a medical-research point of view.)
Talk to your doctor about it. It may be that if you were just on the borderline of high blood pressure but have special risks for cancer, she may want you off the pills. But if your doctor feels your risks from uncontrolled high blood pressure outweigh the possible slightly increased risks of cancer turned up by this study, I’d trust him. In the unlikely chance your doctor doesn’t know what you’re talking about when you ask about this study, I myself would tell him that I’d be happy to sit there and wait while he looked the study up.
David, I'm confused. When you define them for those of us without a statistics background, you say that the absolute risk is a big difference, and the relative risk a small difference. Then right after that you refer to the press emphasizing the relative risk, a "generally misleadingly high number," and then talk about the "absolute risk, which tends to be a very low number." And then you give my hypothetical doctor two different genders in the same sentence.
Hi Carol, glad you asked. The gender thing is kind of odd, I admit, but I've adopted my book publisher's policy of switching genders on just about every reference that doesn't refer to a specific person. I know it's "your" doctor, but I meant "you" as in the different doctors of every different person reading the post. OK, you're right, it's awkward.It's true, relative risk is generally a misleadingly high number–if there's given rise in risk, it will be a higher number, and usually much higher, if expressed as relative risk rather than absolute risk. So why did my example involve relative risk being lower? It didn't–in my example I was actually showing how a relative risk increase of 10% would translate to an absolute risk of only about 1% (in that particular case). But I didn't make that very clear, did I? I'll see if I can edit the post to avoid that confusion. It's actually kind of hard to quickly and clearly get across the idea of absolute vs. relative risk–which is why I think experts and the media ought to stick with absolute risk in most cases.
I’m not sure if I read over it, but what is this review study called? I’m interested to read more now that you’ve given me a taste! Thanks!
Hello Charis, it’s called “Associations of Kidney Function with Cardiovascular Medication Use after Myocardial Infarction.” Enjoy!