Randomised Controlled Trial
A study where people are randomly assigned to treatments — the strongest test of whether something works.
What it means
A study in which people are allocated at random to receive one treatment or another, or a placebo.
Randomising is the whole point. It means the groups are alike in every respect except the treatment — including in ways nobody thought to measure — so any difference in outcome can be attributed to the treatment itself.
"Double-blind" means neither the participants nor the researchers know who got what, which removes another layer of bias.
Why it matters: observational studies, where researchers simply watch what people already do, repeatedly produce findings that randomised trials then overturn. People who take vitamins are healthier than people who do not — but trials of vitamins mostly show no benefit, because the vitamins were never the reason.
In practice
Underpinning every guideline recommendation worth having.
A systematic review or meta-analysis pooling several RCTs sits above a single trial in the strength of what it can tell you.
Why it matters
When you read a health claim, ask what kind of study it came from. "Linked to" and "associated with" almost always signal an observational study, which cannot establish cause.
When considering a paid treatment, ask what randomised evidence supports it. That question separates most of what works from most of what is sold — and it is the question we would want you to ask us.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
August 30, 2026