"Cuts your risk by 53%." "Doubles your chance of heart disease." Headlines like these grab attention because the numbers sound enormous. Sometimes they are. Often, though, the real-world change is much smaller than it sounds. The difference comes down to two kinds of risk: relative and absolute. Once you see how they work, you will never read a health headline the same way again.
A simple example
Imagine 1,000 people who do not take a supplement. Over five years, 4 of them develop a certain condition. Now imagine 1,000 similar people who do take the supplement. In that group, 2 develop the condition.
- Relative risk reduction: the risk went from 4 to 2, which is a 50% drop. A headline could say "Supplement cuts risk in half."
- Absolute risk reduction: the risk went from 0.4% to 0.2%, a difference of 0.2 percentage points. That means 2 fewer cases for every 1,000 people over five years.
Both statements are true. But they feel very different. One sounds like a breakthrough. The other sounds like a small benefit for a rare problem.
Why it matters
The absolute number tells you how much your own situation is likely to change. If a condition is rare, even a big relative reduction means a small absolute benefit. If a condition is common, a modest relative reduction can matter a lot.
This applies to supplements, medicines, diets and lifestyle changes alike. It also applies to scary headlines. "Doubles your risk" can mean going from 1 in 10,000 to 2 in 10,000.
Number needed to treat
Researchers often use a measure called the number needed to treat. It answers a simple question: how many people need to take this for one person to benefit?
In our example, 2 fewer cases per 1,000 people means about 500 people would need to take the supplement for five years to prevent one case. That is useful information when you are weighing cost, side effects and effort.
To work it out yourself, divide 100 by the absolute risk reduction in percentage points. Here, 100 divided by 0.2 equals 500.
Questions to ask when you see a big percentage
- Percent of what? Look for the starting risk. If the article does not give it, be cautious.
- Over what time period? A risk over 20 years is different from a risk over one year.
- In whom? A benefit seen in high-risk people may be much smaller for healthy people.
- What kind of study? A pattern seen in an observational study is not the same as a result from a randomized trial. See how to read a health headline.
- What are the downsides? A small absolute benefit may not be worth significant side effects or cost.
A real-world example with supplements
Large trials of vitamin D and omega-3 supplements in healthy adults have generally found small or no reductions in major outcomes like heart attacks and cancer. When a subgroup shows a benefit, it is often reported as a relative reduction, which can make it sound larger than it is. Reading the absolute numbers helps keep it in perspective. We discuss this in why omega-3 trial results look mixed.
Relative risk is not useless
Relative risk is a perfectly good way to describe how strong an effect is, and it helps scientists compare results across studies. The problem is only when it is presented without the absolute numbers. Good health reporting gives you both.
If you want to dig into how risk is communicated, the US National Cancer Institute and many university health sites have short, clear explainers. The NIH also offers an accessible guide to understanding health research.
The bottom line
Relative risk tells you how much something changes compared with the starting point. Absolute risk tells you how much it changes in real life. When a headline uses a big percentage, look for the starting risk, the time period and the number of people who actually benefit. It is the best defense against both hype and needless fear.