Absolute Risk vs Relative Risk in Drug Side Effects: How to Interpret Numbers

Have you ever read a drug ad that says, "This medication cuts your risk of heart attack in half"? It sounds impressive-until you realize that your actual risk went from 2% to 1%. That’s not a miracle cure. It’s a 1 percentage point change. But most people don’t know the difference between absolute risk and relative risk. And that’s exactly how pharmaceutical companies get away with making side effects and benefits sound far more dramatic than they are.

What’s the Difference Between Absolute and Relative Risk?

Absolute risk tells you the real, plain number: how likely something is to happen to you. If 1 out of every 100 people who take a drug gets a serious side effect, the absolute risk is 1%. Simple. Concrete. No tricks.

Relative risk, on the other hand, compares two groups. It’s a ratio. If people on a drug have half the risk of a heart attack compared to those not taking it, that’s a 50% relative risk reduction. Sounds powerful. But if the baseline risk was only 2%, then the actual drop is just 1 percentage point-from 2% to 1%. The math is correct, but the impact? Tiny.

Here’s how the numbers break down:

• Absolute Risk Reduction (ARR) = Control Group Risk − Treatment Group Risk
• Relative Risk Reduction (RRR) = (Control Group Risk − Treatment Group Risk) ÷ Control Group Risk

Take statins, for example. A study shows that over 5 years, 2% of people not taking statins had a heart attack. Among those taking statins, it was 1%. The absolute risk reduction is 1% (2% − 1%). The relative risk reduction? 50% (1% ÷ 2% = 0.5). Same data. Two very different stories.

Why Do Ads Use Relative Risk?

Because bigger numbers sell better.

A drug that reduces cancer risk from 0.001% to 0.00001% sounds useless when you say it’s a 0.00099% absolute drop. But say it’s a 99% relative risk reduction? Suddenly, it sounds life-saving. That’s not a mistake-it’s a strategy.

According to a 2021 study in the Journal of General Internal Medicine, 78% of direct-to-consumer drug ads in the U.S. highlight relative risk reduction without ever mentioning the absolute numbers. The FDA doesn’t require it. So companies don’t volunteer it. Why? Because patients are more likely to say yes to a treatment that "cuts risk in half" than one that "lowers your chance from 2% to 1%."

And it works. Market research from IQVIA shows drugs marketed with relative risk reductions get 23% more initial prescriptions. But here’s the catch: when patients later find out the real numbers, adherence drops by 15%. They feel misled. And that’s not just frustrating-it’s dangerous.

When Absolute Risk Matters Most

Let’s say you’re considering a new antidepressant. The ad says, "Patients on this drug are 2.4 times more likely to experience sexual dysfunction." That sounds scary. But if 8.3% of people on placebo had it, and 20% on the drug did, the absolute increase is just 11.7 percentage points. So, 1 in 8.5 people-not half, not most, but a minority-will have this side effect.

Now imagine a patient with a 0.75% lifetime risk of cancer. After radiation exposure, their risk rises to 1.25%. The relative increase? 67%. That’s what the news headline says. But the absolute increase? Just 0.5 percentage points. That’s not a disaster. It’s a small, measurable change. But without context, fear wins.

That’s why absolute risk is critical. It tells you whether the change is meaningful in your life. A 10% relative reduction in stroke risk sounds great. But if your baseline risk is 0.1%, the absolute reduction is 0.01%. That’s 1 in 10,000. Is the drug worth the cost, the side effects, the daily pill? Maybe not.

What About Number Needed to Treat (NNT)?

There’s another number doctors use to cut through the noise: Number Needed to Treat, or NNT.

NNT = 1 ÷ Absolute Risk Reduction (as a decimal)

If a drug reduces heart attack risk by 1% (0.01), then NNT = 1 ÷ 0.01 = 100. That means 100 people need to take the drug for one person to avoid a heart attack. The other 99? They get no benefit. But they still face side effects, costs, and potential harm.

Compare that to a drug with an NNT of 5. That’s a much stronger signal. One in five people benefits. That’s worth considering.

NNT doesn’t show up in ads. But it’s one of the most useful tools for real decision-making.

How Patients Get Confused-and Why It Matters

On Reddit, a user named u/PrimaryCareDocMD shared this story: "I had a patient refuse statins because they read online that it "cuts heart attack risk in half." They didn’t realize their risk went from 2% to 1%. They thought it meant half the people wouldn’t have heart attacks."

That’s not a rare mistake. A 2019 study in JAMA Internal Medicine found 60% of physicians couldn’t correctly convert a relative risk reduction into absolute terms. If doctors struggle, how can patients be expected to understand?

And it’s not just about statins. A 2023 thread on Patient.info had dozens of comments from people who thought "50% reduction in side effects" meant "half of people won’t get side effects." They didn’t realize it meant their personal chance dropped from 20% to 10%.

When patients misunderstand risk, they make poor choices. They skip lifesaving drugs because they think the benefit is tiny. Or they take risky drugs because they think the benefit is huge. Either way, outcomes suffer.

How to Read Risk Numbers Like a Pro

Here’s how to cut through the noise:

1. Find the baseline. What’s the risk without the drug? If it’s not stated, ask. "What’s my risk if I don’t take this?"
2. Calculate the absolute change. Subtract the treatment risk from the control risk. That’s the real-world impact.
3. Ask for NNT. How many people need to take it for one to benefit? If it’s over 50, the benefit is small for most.
4. Ignore relative percentages alone. A "50% reduction" could mean anything. Always pair it with the starting number.
5. Look for visuals. Charts showing 100 people with X affected are far more understandable than percentages.

For example, if a drug reduces your risk of a rare side effect from 1 in 10,000 to 1 in 100,000, the relative risk reduction is 90%. But the absolute reduction? 0.009%. That’s not meaningful for most people. The benefit? Tiny. The risk? Still there.

What’s Changing? Better Rules Are Coming

In January 2023, the FDA released draft guidance requiring clearer risk communication in drug ads. They’re finally acknowledging the problem: relative risk alone misleads people.

Europe already requires both absolute and relative risk in patient leaflets. Harvard Medical School now teaches risk interpretation in its core curriculum. And by 2025, 90% of clinical trial registries will have to report both metrics.

But until then, you’re on your own. Don’t trust an ad. Don’t trust a headline. Don’t trust a doctor who only gives you percentages without context.

Ask for the numbers. Ask for the baseline. Ask for the NNT. If they can’t-or won’t-give them to you, that’s a red flag.

Bottom Line

Relative risk tells you how much better or worse something is compared to something else. Absolute risk tells you what actually happens to you.

One is a ratio. The other is reality.

When it comes to drugs, side effects, and your health, always start with the absolute. The rest is just noise.