How to read medical evidence.

The single most useful skill for navigating longevity medicine — or any medicine — is the ability to weigh evidence. It is not as difficult as it sounds.

Not all evidence is equal

Evidence exists on a hierarchy. At the stronger end: large randomized controlled trials and careful syntheses of many trials. In the middle: observational studies, which can show association but struggle to prove causation. At the early end: preclinical work in cells and animals — valuable for generating hypotheses, but not proof of human benefit. A claim is only as strong as the evidence tier beneath it.

Three questions to ask

• What kind of study is this? A randomized trial and a mouse study support very different levels of confidence.
• Studied in whom? Findings in one population may not transfer to you. Animal results may not transfer to humans at all.
• How large is the effect — and how certain? A real but tiny effect may not be worth acting on. “Statistically significant” is not the same as “significant for you”.

Watch for the tells

Be cautious with claims that lean on a single study, that cite only preclinical work while implying human benefit, that report relative changes without absolute ones, or that are made by someone selling the product. None of these is automatically disqualifying — but each is a reason to look closer.