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

Imagine you see an ad for a new medication. It says, "Reduces heart attack risk by 50%." That sounds impressive, right? You might think, "This drug cuts my chance of a heart attack in half." But what if your actual risk was only 2% to begin with? After taking the drug, it drops to 1%. That’s still a 50% relative reduction-but only a 1 percentage point absolute change. Most people don’t realize the difference between those two numbers, and pharmaceutical companies count on that.

What’s the difference between absolute and relative risk?

Absolute risk tells you the real, everyday chance of something happening to you. It’s a straightforward number: how many people out of 100, 1,000, or 100,000 actually experience a side effect or benefit. For example, if 2 out of every 100 people have a heart attack in a year, their absolute risk is 2%.

Relative risk compares two groups. It’s a ratio: how much more or less likely one group is to experience an event compared to another. If a drug lowers heart attack risk from 2% to 1%, the relative risk reduction is 50% because 1% is half of 2%. That sounds big. But the absolute risk reduction is just 1%-the actual difference between 2% and 1%.

This isn’t just math. It’s about real decisions. A 50% relative risk reduction sounds like a miracle. A 1% absolute reduction sounds like a small benefit. Both are true. But only one tells you what actually matters for your body.

Why do drug ads use relative risk?

Because it makes the numbers look better.

Take a drug that reduces the risk of a rare side effect from 1 in 100,000 to 1 in 10,000,000. That’s a 99% relative risk reduction. Sounds amazing. But the absolute risk reduction? Just 0.099%. You’re talking about preventing one case in nearly 100,000 people taking the drug. That’s not nothing-but it’s not a game-changer either.

A 2021 study found that 78% of direct-to-consumer drug ads in the U.S. highlight relative risk reduction without ever mentioning the absolute number. Why? Because 90% relative risk sounds way more compelling than 0.099%. It’s not lying. It’s selective storytelling.

The same trick works for side effects. A drug might double your risk of nausea. That’s a 100% relative increase. But if the baseline risk is 5%, and it goes up to 10%, that’s only a 5 percentage point absolute increase. Half of all people taking the drug still won’t feel sick. The relative number scares you. The absolute number tells you it’s manageable.

How to calculate both types of risk

You don’t need to be a statistician to understand this. Here’s how it breaks down:

• Absolute Risk Reduction (ARR) = Risk in control group − Risk in treatment group
• Relative Risk Reduction (RRR) = (ARR ÷ Risk in control group) × 100%

For example:

- Control group: 10% of people have a stroke - Treatment group: 8% have a stroke - ARR = 10% − 8% = 2% - RRR = (2% ÷ 10%) × 100% = 20%

The drug reduces stroke risk by 20% relative to the control group. But you personally only lowered your chance by 2 percentage points. That’s the real impact.

You can also calculate the Number Needed to Treat (NNT)-how many people need to take the drug for one person to benefit. Just divide 1 by the ARR (as a decimal). In this case: 1 ÷ 0.02 = 50. So, 50 people must take the drug for one stroke to be prevented. That tells you whether the benefit is worth the cost, side effects, or hassle.

Why absolute risk matters more for patients

Doctors and researchers need both numbers. But patients need to know what it means for them.

A 2019 study in JAMA Internal Medicine found that 60% of physicians couldn’t correctly convert a relative risk reduction into an absolute one. If the people giving you advice don’t fully get it, how can you?

Consider this real story from a Reddit thread: a patient refused statins because they read online that the drug "cuts heart attack risk in half." They didn’t know their baseline risk was 2%. After taking statins, it dropped to 1%. The patient thought "half" meant they’d go from likely to unlikely. In reality, they went from unlikely to still unlikely-but slightly less so.

When patients understand absolute risk, they make better choices. One woman in a medical journal case study was ready to skip a life-saving treatment because she thought the relative risk reduction sounded too small. When her doctor showed her the absolute numbers-her risk dropped from 15% to 12%-she realized the benefit was worth it. That 3% difference meant 1 in 33 people like her would avoid a bad outcome. That’s meaningful.

How to spot misleading risk claims

Here’s a quick checklist to use when you see a drug claim:

1. Find the baseline risk. What’s the chance of the problem happening without the drug? If it’s not stated, ask. A 50% reduction means nothing if the starting risk is 0.1%.
2. Ask for the absolute difference. Not the percentage. The actual number. "How many fewer people have the problem?"
3. Check the time frame. Is the risk reduction over 1 year? 5 years? 10? A 20% reduction over 10 years is very different from one over 1 year.
4. Look for the Number Needed to Treat (NNT). If it’s over 50, the benefit is small for most people. If it’s under 10, it’s significant.
5. Ask about side effects. What’s the absolute risk of those? A drug might reduce heart attacks by 2% but increase severe muscle pain by 5%. That’s a trade-off.

Pharmaceutical companies aren’t evil. They’re following the rules. But the rules let them show the most exciting number, not the most useful one. You have to dig deeper.

What experts say about this

Dr. Steve Woloshin and Dr. Lisa Schwartz from the Dartmouth Institute have spent decades studying how medical numbers are presented. Their research shows that when patients are given absolute risks with visual aids-like pictures of 100 people, with some shaded to show who benefits or gets side effects-62% understand the risk. Without visuals, only 8% get it right.

The Cochrane Collaboration, a respected global network of medical researchers, now requires journals to use standardized risk communication templates. They recommend always showing absolute risk first, then relative risk with clear context.

In 2023, the FDA released draft guidance pushing for clearer risk reporting in ads. They called out the "potential for misleading interpretations" when only relative risks are shown. But enforcement is still weak. Most ads haven’t changed.

What you should do next

When your doctor talks about a new medication, don’t just nod along. Ask:

• "What’s my risk of this problem without the drug?"
• "How much does it lower that risk in real numbers?"
• "How many people like me would need to take this for one to benefit?"
• "What’s the absolute risk of the side effects?"

Write it down. Compare it. If they can’t answer, ask for a handout or look up the trial data yourself. Most clinical trials publish this information.

You don’t need to be an expert to understand your health. You just need to ask the right questions. The numbers are there. You just have to know how to read them.

Common examples you’ll see

Here are real-world comparisons to help you think clearly:

Comparing Absolute and Relative Risk in Common Drug Scenarios

Scenario	Baseline Risk (Control)	After Treatment	Absolute Risk Reduction	Relative Risk Reduction	Number Needed to Treat (NNT)
Statin for heart attack prevention	2%	1%	1%	50%	100
Antidepressant for depression	30%	20%	10%	33%	10
Drug reducing rare cancer risk	0.01%	0.005%	0.005%	50%	20,000
Drug increasing nausea risk	5%	10%	+5%	+100%	N/A

Notice how the same relative reduction (50%) can mean wildly different things. One prevents one heart attack per 100 people. The other prevents one cancer case per 20,000. One is meaningful. The other is barely noticeable.

What to remember

- Absolute risk = what happens to you. Relative risk = what happens compared to someone else. - Big relative numbers often hide tiny real-world benefits. - Side effects sound scarier with relative risk. Always ask for the absolute number. - NNT tells you if the benefit is worth it. Under 10? Strong. Over 50? Weak. - Visuals help. Ask for charts or diagrams showing 100 people. - You have the right to know both numbers. Don’t accept one without the other.

The goal isn’t to scare you off medicine. It’s to help you make smart choices. Some drugs save lives. Others barely move the needle. The numbers tell you which is which-if you know how to read them.