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

Have you ever seen a drug ad that says, "This medication cuts your risk of heart attack in half"? It sounds powerful - like you’re doing something huge for your health. But what if I told you that your actual risk only went from 2% to 1%? That’s still a 50% reduction - but it’s not the life-saving leap it sounds like. This is the gap between absolute risk and relative risk, and understanding the difference can change how you think about every pill you take.

What Absolute Risk Really Means

Absolute risk is the real, honest number: how likely something is to happen to you, right now, in your life. It doesn’t compare you to anyone else. It just says: out of 100 people like you, how many will have this side effect?

For example, if a drug causes severe nausea in 3 out of 100 people, the absolute risk is 3%. That’s it. No tricks. No math games. Just facts. If you’re one of those 100, you have a 3% chance of feeling sick. If you’re not, you’re fine. Simple.

This is what matters most when you’re deciding whether to take a drug. If the side effect is rare - say, 1 in 10,000 - then even if it’s serious, the odds are low. But if it’s 1 in 10? That’s a different conversation.

Doctors use absolute risk to make real decisions. When prescribing statins for high cholesterol, they don’t just say, "This lowers your risk." They say, "For someone your age with your numbers, this drug reduces your chance of a heart attack from 10% to 8% over 10 years." That’s absolute risk. It’s grounded. It’s personal.

What Relative Risk Is (and Why It’s Misleading)

Relative risk is a ratio. It compares your risk on the drug to your risk without it. It doesn’t tell you the starting point. That’s the problem.

Take a drug that reduces heart attack risk from 2% to 1%. The absolute risk reduction? Just 1 percentage point. But the relative risk reduction? 50%. That’s half. And that’s what ads love to shout.

Here’s another example: a drug reduces the chance of a rare side effect from 1 in 100,000 to 1 in 1,000,000. That’s a 90% relative risk reduction. Sounds amazing. But in real terms? You went from a 0.001% chance to a 0.00001% chance. You’re talking about one extra person out of every 111,000 who avoids the side effect. That’s not nothing - but it’s not a miracle either.

Pharmaceutical companies use relative risk because it makes their drugs look better. A 50% reduction sounds way more impressive than a 1% reduction. And let’s be honest - if you don’t know the difference, you’ll believe the bigger number.

Studies show that 78% of direct-to-consumer drug ads in the U.S. use relative risk without ever showing the absolute numbers. That’s not an accident. It’s strategy.

Why Both Numbers Matter - Together

You can’t understand a drug’s effect with just one number. You need both.

Let’s say a new antidepressant increases the risk of sexual side effects from 8% on placebo to 20% on the drug. The absolute increase? 12 percentage points. That’s 1 in 8 people. Pretty significant.

The relative risk? 2.5 times higher. That sounds scary - and it should. But if you only heard "2.5 times higher," you might think it affects half the people. It doesn’t. It affects 12% more. Still a lot - but not half.

On the flip side, if a drug reduces stroke risk from 0.5% to 0.4%, the relative reduction is 20%. Sounds small. But if you’re at high risk for stroke, that 0.1% drop might mean avoiding a life-changing event. Context changes everything.

That’s why experts say: always ask for both. If a doctor says, "This drug reduces your risk by 30%," ask: "From what? To what?" That’s the key.

The Number Needed to Treat (NNT) - A Real-World Measure

There’s another number that cuts through the noise: Number Needed to Treat, or NNT.

NNT tells you how many people need to take a drug for one person to benefit. It’s calculated from absolute risk reduction. If a drug reduces heart attack risk by 2% (0.02), then NNT = 1 / 0.02 = 50. That means 50 people need to take it for one person to avoid a heart attack.

That’s not a bad thing. But it does show you the scale. If the drug has side effects that affect 1 in 10 people, you’re giving a side effect to 5 people for every one who benefits. That’s a trade-off.

NNT is used in clinical guidelines for a reason. It forces you to think in real numbers, not percentages. If NNT is 100, and side effects affect 1 in 5, you might decide it’s not worth it. If NNT is 5 and side effects are rare? That’s a different call.

How Ads and Media Twist the Numbers

You’ve probably seen headlines like: "New Drug Cuts Cancer Risk by 70%!"

That’s almost always relative risk. The absolute risk? Maybe it went from 0.75% to 1.25% - a 0.5 percentage point increase. That’s a 70% relative increase, but the real-world change? Tiny. This happened during the Fukushima nuclear disaster. News outlets said cancer risk "increased 70%" - terrifying. But the actual increase? From 0.75% to 1.25%. Still less than 1.5% total risk.

Same with drugs. A statin ad says, "Reduces heart attack risk by 50%." But if your baseline risk is 1%, it’s now 0.5%. You’re still at risk. You just have half the chance. That’s not a cure. It’s a modest improvement.

And here’s the kicker: patients who only hear relative risk are more likely to start a drug - and more likely to stop it later when they realize the benefit was smaller than they thought. One study found that 60% of doctors couldn’t even convert relative risk into absolute terms. If they’re confused, how can patients understand?

What You Should Do

If you’re considering a new medication, here’s what to ask:

• What’s my baseline risk without this drug?
• What’s my risk with it?
• What’s the absolute difference?
• How many people need to take this for one to benefit? (NNT)
• What are the real odds of side effects? Not "more likely," but "how many out of 100?"

Don’t be afraid to ask for numbers. If your doctor says, "It’s 50% better," say: "Can you show me the numbers?"

Visual tools help. Some clinics use charts with 100 little icons - 100 people. Color the ones who get better, the ones who get side effects. Suddenly, it’s clear. A 10% improvement isn’t a miracle. It’s 10 out of 100.

And if you’re reading an ad? Look for the fine print. If it doesn’t mention the starting risk, it’s hiding something.

What’s Changing Now

Regulators are waking up. In 2023, the FDA started pushing for clearer risk communication in drug ads. The European Medicines Agency already requires both absolute and relative numbers on patient leaflets. Harvard Medical School now teaches risk interpretation in its curriculum. Cochrane, a global health research group, has created standardized templates so journals report data the same way.

But change is slow. Most patients still don’t know the difference. And the industry still has every reason to keep using relative risk - because it works. It sells.

So you have to be the one who asks. Who digs deeper. Who doesn’t let a 50% number scare or excite you without knowing the full story.