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

Imagine you’re told a new medication cuts your risk of a heart attack in half. That sounds amazing - until you find out your original risk was only 2%. Halving that means your new risk is 1%. That’s not a miracle drug. It’s a small benefit hidden behind a big number. This is the trap most people fall into when they hear about drug side effects or benefits. The difference between absolute risk and relative risk isn’t just math - it’s the difference between making an informed choice and being misled.

What Absolute Risk Really Means

Absolute risk tells you the actual chance of something happening to you. It’s simple: out of 100 people like you, how many will experience the side effect or benefit? If a drug causes liver damage in 1 out of every 1,000 people, that’s an absolute risk of 0.1%. If it prevents a stroke in 5 out of 100, that’s a 5% absolute benefit. No tricks. No percentages hiding behind other numbers.

This is the number that matters most when you’re deciding whether to take a pill. It answers the question: What’s the real impact on my body? A 0.1% chance of liver damage means you’d need to treat 1,000 people before one person has a problem. That’s not common. But if the drug reduces your stroke risk from 10% to 5%, that’s a 5% absolute benefit - meaning 1 in 20 people will avoid a stroke. That’s meaningful.

Doctors and regulators use absolute risk to calculate the Number Needed to Treat (NNT). If a drug has a 5% absolute benefit, the NNT is 20. That means 20 people need to take it for one person to benefit. If the NNT is 100, you’re treating 100 people for one good outcome. That’s a very different picture than hearing it "reduces risk by 50%."

What Relative Risk Tells You (and Doesn’t)

Relative risk compares two groups: people who took the drug versus those who didn’t. It’s a ratio. If the risk of a side effect is 2% in the control group and 4% in the drug group, the relative risk is 2.0 - meaning you’re twice as likely to have the side effect. That sounds scary. But here’s the catch: 2% to 4% is still only a 2 percentage point increase. That’s an absolute risk increase of 2%. Two out of 100 people. Not everyone.

Pharmaceutical ads love relative risk because it makes small changes look huge. A drug that reduces heart attack risk from 2% to 1% sounds like a 50% reduction. That’s a 50% relative risk reduction. But the absolute benefit? Just 1 percentage point. One in 100 people benefits. The other 99 get no protection - but still face the same side effects.

Here’s a real example from a statin study: The relative risk reduction for heart attack was 30%. Sounds impressive. But the absolute risk reduction? Just 1.2%. That means for every 100 people taking statins, only 1 or 2 avoid a heart attack. The rest? They’re exposed to muscle pain, diabetes risk, and liver enzyme changes - all for a tiny chance of benefit. If you don’t know the absolute numbers, you think it’s a miracle. It’s not.

Why the Difference Matters So Much

The gap between absolute and relative risk isn’t a technicality - it’s a communication failure. And it’s costing people money, health, and peace of mind.

Take the case of a woman in Brisbane who refused a blood pressure drug because the ad said it "reduced stroke risk by 40%." She thought it meant she’d have a 60% lower chance of stroke. But her baseline risk was only 0.5%. The drug brought it down to 0.3%. That’s a 40% relative reduction - but only a 0.2 percentage point absolute drop. She’d need to take it for 500 years to prevent one stroke in her lifetime. She didn’t need the drug. She needed the truth.

Another example: A new antidepressant increases the risk of sexual dysfunction by 2.4 times compared to placebo. That sounds alarming. But if 8% of placebo users had the side effect, and 20% of drug users did, the absolute increase is just 12 percentage points. That’s 12 out of 100 people. Still common - but not a rare, freak accident. Context changes everything.

Without absolute numbers, you can’t judge if the trade-off is worth it. Is a 2% increase in diabetes risk worth a 1% reduction in heart attacks? You can’t answer that if you only know the relative risk.

How the Industry Uses This to Sell Drugs

The pharmaceutical industry doesn’t hide this trick by accident. It’s deliberate. A 2021 study found that 78% of direct-to-consumer drug ads in the U.S. used relative risk reductions - and didn’t mention absolute risk at all. Why? Because 50% sounds better than 1%. 90% sounds like a cure. But 0.099%? That’s a whisper.

One drug advertised a 90% reduction in cancer risk after a nuclear accident. The absolute risk? Went from 0.75% to 1.25%. That’s a 0.5 percentage point increase - not a 90% danger. But the headline? "Cancer Risk Skyrockets 90%." That’s fear-driven marketing. And it works.

Marketing teams know that people don’t understand statistics. They also know that fear and hope drive behavior more than facts. So they give you the flashy number - the relative risk - and let you fill in the blanks. Most people assume "50% reduction" means half of people are saved. It doesn’t. It means your personal risk was cut in half - from something small to something even smaller.

How to Read the Numbers Like a Pro

You don’t need a degree in biostatistics to cut through the noise. Here’s how to decode any drug claim:

1. Find the baseline risk. What’s the chance of the problem happening without the drug? Look for phrases like "in people like you" or "among those with high blood pressure." If it’s not stated, ask.
2. Find the absolute benefit or harm. Ask: "How many fewer people had the outcome?" or "How many more had the side effect?" If they only give you percentages like "50% reduction," ask for the actual numbers.
3. Calculate the NNT or NNH. If the absolute benefit is 5%, the NNT is 20. If the absolute harm is 3%, the Number Needed to Harm (NNH) is 33. That tells you how many people you need to treat to see one benefit - or one harm.
4. Compare the NNT to the NNH. If the NNT is 10 and the NNH is 50, the benefit outweighs the risk. If the NNT is 100 and the NNH is 20, you’re doing more harm than good.

For example: A drug reduces migraine frequency by 50%. Sounds great. But if you have 4 migraines a month, and the drug brings it down to 2, that’s a 50% relative reduction. But the absolute reduction? Just 2 migraines a month. Is that worth the cost, the side effects, and the daily pill? Maybe. But you can’t know unless you see the real numbers.

What You Should Ask Your Doctor

Don’t let your doctor skip this part. Ask these questions:

• "What’s my risk of this problem without the drug?"
• "How much does the drug actually lower that risk?"
• "What’s the chance I’ll have a side effect?"
• "How many people like me need to take this for one person to benefit?"
• "How many need to take it before one person has a bad reaction?"

If they say, "It cuts your risk in half," ask: "Half of what?" Then write down the numbers. Don’t trust memory. Write it down.

Good doctors will show you charts with 100 little people - 100 dots representing you and others. Some are colored red for side effects, green for benefit. You can see it visually. That’s the gold standard. If they don’t use visuals, ask for them.

The Bottom Line: Numbers Don’t Lie - But People Do

Absolute risk tells you what’s real. Relative risk tells you how it’s sold. You need both - but if you only get one, make it absolute. A 90% relative risk reduction sounds like a miracle. But if your baseline risk was 0.001%, then the absolute reduction is 0.0009%. You’re talking about one person in a million. That’s not a miracle. It’s a statistical ghost.

On the other hand, a 10% absolute benefit? That’s 1 in 10 people helped. That’s real. That’s worth talking about.

Next time you see a drug ad, a study, or your doctor mention a "reduction," pause. Ask for the numbers. Write them down. Compare them. Your health isn’t a marketing campaign. It’s your life.

What Happens When You Get It Right

There’s a reason the Cochrane Collaboration and the FDA now push for absolute risk reporting. When patients understand the real numbers, they make better decisions. One study found that when patients saw both absolute and relative risks with visual aids, 62% understood the benefit. When they only saw relative risk? Only 8% got it right.

Patients who understood absolute risk were more likely to stick with treatments that truly helped them - and more likely to stop ones that didn’t. That’s not just smarter. It’s safer.

By 2025, most clinical trials will be required to report both absolute and relative risks. That’s progress. But until then, you have to be the one who asks. You have to be the one who reads between the numbers.