GlossaryEvidence & ResearchRCT (Randomized Controlled Trial)
Evidence & Research

RCT (Randomized Controlled Trial)

Also known as: RCT · randomized controlled trial

A randomized controlled trial (RCT) assigns people by chance to a treatment or comparison group, so outcome differences can be credited to the treatment.

In an RCT, a random process, not the researcher or the participant, decides who gets the intervention and who gets the comparator, usually a placebo or an existing treatment. Randomization balances known and unknown factors between the groups on average, so a difference in outcomes is more likely to come from the intervention than from who chose it. Blinding (keeping participants and assessors unaware of who got what) reduces bias from expectations, and a primary outcome named in advance keeps researchers from picking whichever result looks best.

RCTs sit near the top of the traditional evidence pyramid, below systematic reviews that pool them, and under GRADE randomized trials start as high-certainty evidence. They can still be rated down for risk of bias, imprecision, inconsistency, indirectness or publication bias. The CONSORT statement sets out what a trial report should include; the 2025 version is a 30-item checklist plus a diagram of how participants moved through the trial.

Why it matters

RCTs are how logical-sounding treatments get tested. In the Cardiac Arrhythmia Suppression Trial (CAST), drugs that suppressed abnormal heartbeats after a heart attack were expected to prevent sudden death. Randomized against placebo, encainide and flecainide instead raised total mortality (56 of 730 vs 22 of 725), and that arm was stopped. Without a randomized control group, extra deaths in treated patients could easily have been blamed on how sick they already were.

When you see "studies show" attached to a peptide, check whether any of those studies randomized people, and whether the people, dose and outcome match what is being claimed. In DoseCraft's methodology, randomized or controlled human trials are among the evidence types that can place a claim in the Clinical lane.

How to read an RCT

  1. Check how participants were randomized and whether the next assignment was hidden from the people enrolling them.
  2. Check who was blinded: participants, clinicians, outcome assessors.
  3. Find the primary outcome named in advance and its result before looking at secondary or subgroup findings.
  4. Use the flow diagram to see how many people dropped out of each group, and why.
  5. Convert relative risks into absolute differences.

Absolute risk difference = risk in treated group - risk in control group; number needed to harm (or treat) = 1 / absolute risk difference

Worked example

In CAST, 56 of 730 patients on encainide or flecainide died (7.67%) versus 22 of 725 on placebo (3.03%). The absolute difference is 7.67% - 3.03% = 4.64 percentage points, and 1 / 0.0464 = 21.6, so roughly one extra death for every 22 patients treated over an average of 10 months. The trial reported this as a relative risk of 2.5.

Common mistakes

  • Treating "randomized" as a guarantee of quality; small, unblinded or high-dropout trials can still be biased.
  • Reading a relative risk without the absolute numbers.
  • Focusing on a subgroup or secondary outcome when the primary outcome was negative.
  • Assuming an RCT of one drug, dose or population applies to a different one.
  • Treating an animal study described as "randomized" as equivalent to a human RCT.

Sources

  1. GuidelineGRADE guidelines: 3. Rating the quality of evidence. J Clin Epidemiol, 2011. PMID 21208779.
  2. GuidelineCONSORT 2025 statement: updated guideline for reporting randomised trials. Lancet, 2025. PMID 40245901.
  3. Clinical trialPreliminary report: effect of encainide and flecainide on mortality in a randomized trial of arrhythmia suppression after myocardial infarction. N Engl J Med, 1989. PMID 2473403.

Page updated . How we grade evidence: methodology.

Related Terms

Clinical Trial

A clinical trial is a planned study that tests a drug or other intervention in people under a written protocol, usually run in phases 1 through 4.

Preclinical Study

A preclinical study tests a compound in cells or tissue (in vitro) or in living animals (in vivo), before or instead of testing it in people.

Three-Lane Evidence System

The three-lane evidence system is DoseCraft's method of labeling each claim by the strength of evidence behind it: Clinical, Expert or Experimental.

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