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

When a drug ad says it "cuts your risk of heart attack in half," you might think you’re avoiding a 50% chance of having one. But what if your actual risk was only 2% to begin with? After taking the drug, it drops to 1%. That’s not a 50% chance avoided - it’s a 1 percentage point change. This gap between absolute risk and relative risk is why so many people misunderstand drug benefits and side effects - and why doctors, regulators, and patients are finally pushing back.

What Absolute Risk Really Means

Absolute risk tells you the actual chance something will happen to you. It’s simple: out of 100 people like you, how many will experience the side effect? Or how many will avoid a heart attack? It’s measured in real numbers - percentages, per 1,000, or per 100,000.

For example, if 1 in 10,000 people who take a certain blood pressure drug develop a rare liver issue, that’s an absolute risk of 0.01%. If 5 out of 100 people on a statin report muscle pain, that’s a 5% absolute risk. These numbers don’t lie. They’re grounded in your real-world experience.

Doctors and regulators use absolute risk to decide who actually benefits from a drug. If only 1 in 1,000 people gain a meaningful benefit, and 1 in 50 get a side effect, the math says: this drug isn’t for everyone. It’s also why the FDA now recommends absolute risk be shown first in patient materials. Because if you’re trying to decide whether to take a pill for the next 10 years, you need to know: what’s the actual cost to me?

How Relative Risk Can Be Misleading

Relative risk compares two groups - say, people who took the drug versus those who didn’t. It’s a ratio. If the drug reduces heart attacks from 2% to 1%, the relative risk reduction is 50%. That sounds impressive. But here’s the trick: it doesn’t tell you the starting point.

That same 50% relative reduction could mean:

• From 2% to 1% (1 in 100 → 1 in 200) - a small real-world change
• From 50% to 25% (1 in 2 → 1 in 4) - a massive, life-saving drop

Pharmaceutical ads love relative risk because it makes small benefits look huge. A drug that reduces the risk of a rare side effect from 1 in 100,000 to 1 in 1,000,000 sounds like a 90% improvement. But in absolute terms, you’re going from 0.001% to 0.0001% - a 0.0009% change. That’s not meaningful for most people.

One study found 78% of U.S. direct-to-consumer drug ads used relative risk reduction without ever mentioning the baseline risk. Patients walked away thinking they were getting a dramatic benefit - when in reality, their odds barely budged.

Why Both Numbers Matter

You can’t understand a drug’s true impact by looking at just one number. Absolute risk tells you what’s likely to happen to you. Relative risk tells you how much better (or worse) the drug makes things compared to nothing.

Take venlafaxine, an antidepressant. Studies show 20% of people on it report sexual side effects. On placebo, it’s 8.3%. The relative risk is 2.41 - meaning you’re 2.4 times more likely to have this side effect. Sounds scary. But the absolute difference? Just 11.7 percentage points. That’s 1 in 8.5 people. For some, that’s a dealbreaker. For others, it’s worth it to feel better.

Same goes for statins. If you’re a 60-year-old man with high cholesterol and no heart disease, your 10-year risk of a heart attack might be 10%. A statin might lower that to 8%. That’s a 20% relative risk reduction - but only a 2 percentage point absolute drop. For someone with a 2% baseline risk, that same statin cuts it to 1.6% - a 20% relative drop again, but now it’s just 0.4 percentage points. Is that worth daily pills and potential muscle pain? The answer depends on the absolute numbers.

How to Spot the Trap

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. A 50% reduction means nothing if you don’t know what you’re reducing.
2. Look for absolute numbers. If the ad says "reduces risk by 50%," check the small print for "from X% to Y%." If it’s not there, assume the benefit is small.
3. Ask for the Number Needed to Treat (NNT). This tells you how many people need to take the drug for one person to benefit. An NNT of 100 means 99 people take it for no benefit - just one person avoids a bad outcome. That’s not a miracle drug. That’s a gamble.
4. Watch for time frames. "Reduces heart attack risk by 30%" - over 5 years? 10 years? A 30% drop over 10 years is very different from 30% over 1 year.

One patient I spoke with refused a cholesterol drug because she read it "reduced heart attacks by 40%" - until I showed her her personal risk was 1.5%, and the drug lowered it to 0.9%. "That’s not 40% of me having a heart attack," she said. "That’s 1 in 100 becoming 1 in 167. I’m not taking a pill for that." She was right.

What the Experts Say

Dr. Steve Woloshin and Dr. Lisa Schwartz from Dartmouth have spent decades studying how medical numbers are presented. Their research found that only 8% of patients can correctly interpret relative risk alone. But when absolute risk is shown with simple visuals - like a grid of 100 people with colored dots for who gets affected - 62% understand it.

They recommend this simple script:

• "Without the drug, about X out of 100 people like you will have this problem."
• "With the drug, that number drops to Y out of 100."
• "So, the drug helps Z out of 100 people."

That’s it. No percentages, no ratios - just people. That’s how humans think.

Side Effects Are Treated the Same Way

The same math applies to side effects. A drug might increase the risk of depression by 50% - but if the baseline risk is 2%, it only goes up to 3%. That’s a 1 percentage point increase. For someone with no history of depression, that’s not a red flag. For someone already struggling? It’s a serious concern.

Pharmaceutical companies know this. They use relative risk to scare you about side effects when they want to downplay them. But when they want to sell the drug, they use relative risk to make the benefit sound massive. It’s the same tool, flipped.

That’s why the European Medicines Agency now requires both absolute and relative risk in patient leaflets. The U.S. FDA is catching up - but enforcement is still weak. Until then, you have to be the one to ask: "What does this actually mean for me?"

What You Can Do

You don’t need to be a statistician. But you do need to ask the right questions before you take any new medication:

• "What’s my risk of this problem without the drug?"
• "How many people like me actually benefit?"
• "What’s the real chance I’ll get a side effect?"
• "Is this benefit worth the daily pill, cost, and possible side effects?"

Bring a printed copy of the drug’s fact sheet. Ask your doctor to show you the numbers in plain terms. If they can’t or won’t, that’s a red flag. Good care isn’t about selling you a pill - it’s about helping you understand your choices.

Medicine is full of numbers. But your health isn’t a math problem. It’s a personal decision. And you deserve to make it with real information - not inflated percentages.

What Comes Next

If you’re still unsure after checking the numbers, don’t rush. Ask for a second opinion. Look up the drug on the FDA or EMA website - they list clinical trial data. Talk to a pharmacist. They’re trained to read the fine print.

And if you’re on a drug and feel like you were misled - you’re not alone. Millions of people have been. But now you know how to spot it. And next time, you’ll ask the right questions.