When headlines say semaglutide (Wegovy) cut heart attacks and strokes “by 20%”, they are quoting relative risk. In the SELECT trial, the absolute numbers were 6.5% vs 8.0% of people having a major cardiovascular event over about three years – a difference of 1.5 percentage points, meaning roughly 67 people needed treatment for about 3 years to prevent one event. Both numbers are true. Understanding the difference is the single most useful statistics skill for reading longevity news.
The numbers in 30 seconds
- Who: 17,604 adults aged 45+ with heart disease and overweight or obesity, but without diabetes.
- Follow-up: about 40 months on average.
- Heart attack, stroke or cardiovascular death: 6.5% on semaglutide vs 8.0% on placebo.
- Relative risk reduction: about 20%.
- Absolute risk reduction: 1.5 percentage points → about 67 people treated to prevent one event.
- Stopped treatment due to side effects: about 17% vs 8%.
Relative vs absolute risk, explained simply
Imagine 1,000 people like those in the trial. Over about three years:
| Placebo group | Semaglutide group | |
|---|---|---|
| People with a major cardiovascular event | 80 of 1,000 | 65 of 1,000 |
| People without an event | 920 | 935 |
| Events prevented | – | 15 per 1,000 |
Relative risk reduction compares the two groups: 15 fewer events out of 80 is about a 20% drop. Absolute risk reduction is the plain difference: 8.0% minus 6.5% = 1.5 percentage points, or 15 people per 1,000.
The number needed to treat (NNT) turns that into a practical figure: 1,000 ÷ 15 ≈ 67. On average, about 67 people would need to take the drug for roughly three years for one of them to avoid a heart attack, stroke or cardiovascular death.
Is 20% impressive or not?
Both readings are fair. A 20% relative reduction on top of standard heart medication is a meaningful result for a high-risk group, and comparable to some established cardiovascular drugs. At the same time, most people in the trial would not have had an event either way, and the drug comes with costs and side effects.
Relative risk stays similar across groups, but absolute benefit depends on your starting risk. For someone at much lower risk than SELECT participants – for example, a healthy 40-year-old without heart disease – the absolute benefit would be far smaller, and the trial did not test that group.
The other side of the ledger: side effects
In SELECT, about 17% of people on semaglutide stopped the drug because of adverse events, compared with about 8% on placebo – mostly due to gastrointestinal problems such as nausea, vomiting and diarrhoea. Participants on semaglutide lost about 9% of their body weight on average, versus about 1% on placebo; some of that weight loss is lean mass, which is one reason strength training and protein matter when using these drugs.
Four questions to ask about any longevity statistic
- Relative or absolute? “Cuts risk by 50%” can mean 2 in 100 → 1 in 100.
- Who was studied? Results in people with heart disease don’t automatically apply to healthy people.
- What was measured? Hard outcomes (death, heart attack) are more meaningful than markers (a blood value, a “biological age” score).
- What does it cost? Side effects, price and effort belong in the same sentence as the benefit.
How lifestyle numbers compare
The same logic applies to habits. Large observational studies link higher daily step counts and regular strength training with lower mortality – and these come with few side effects. They are not directly comparable to a randomised drug trial, but they belong in the same conversation. See The Minimum Effective Dose of Exercise for Longevity and our movement calculator.
This is not advice to start or stop any medication. GLP-1 drugs are prescription medicines. Decisions about them should be made with your doctor, based on your personal risk.
What to do tomorrow
Next time you read “X reduces risk by Y%”, look for the absolute numbers – usually in the study’s results table or a good news article. Write both down. You’ll be surprised how often the story changes.
Related: Case Study: Can Eating Less Slow Ageing?
Frequently asked questions
What is the difference between relative and absolute risk?
Relative risk compares two groups (e.g. 20% lower); absolute risk is the actual difference in percentage points (e.g. 8.0% vs 6.5% = 1.5 points).
What does number needed to treat mean?
How many people need a treatment for one of them to benefit. In SELECT it was about 67 people over roughly three years.
Should healthy people take semaglutide for longevity?
The SELECT trial studied people with heart disease and overweight or obesity. It did not test healthy people; any decision should be made with a doctor.
Turn this into a habit in 30 days
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Sources
- Lincoff AM et al. Semaglutide and cardiovascular outcomes in obesity without diabetes (SELECT). N Engl J Med, 2023. PubMed
- Akobeng AK. Understanding measures of treatment effect in clinical trials. Arch Dis Child, 2005. PubMed
- Gigerenzer G et al. Helping doctors and patients make sense of health statistics. Psychological Science in the Public Interest, 2007.
This article is for education only and is not medical advice. Read our medical disclaimer.
