[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Epidemiology](https://durgical.com/en/epidemiology/) › [Study designs](https://durgical.com/en/epidemiology/areas/designs/) › Phases of clinical trials

# Phases of clinical trials

- Concept

A new medicine moves through phases: safety in a few volunteers first, then efficacy in hundreds, then thousands of patients and, after approval, monitoring.

Updated Nov 5, 2026

## Key points

- Phase I: safety and dose range; a few dozen healthy volunteers or patients.
- Phase II: early efficacy and adverse effects; up to several hundred patients.
- Phase III: confirms efficacy against the standard; hundreds to thousands of patients; basis for approval.
- Phase IV: after approval; post-marketing surveillance of rare or late effects.
- Many drugs fail in the early phases.
- Rare adverse effects are usually found in phase IV.

## What it is

The clinical development of a medicine moves through phases, each with its own question. Phase I studies safety and the dose range in a few volunteers; phase II explores efficacy and adverse effects in a few hundred patients; phase III confirms efficacy against the standard in hundreds to thousands of patients and supports approval; phase IV takes place after marketing and monitors safety in real practice. The optional phase 0 gives microdoses to very few people to study how the drug behaves in the body.

## Worked example

- Approximate FDA figures: about 70% of drugs move on from phase I, 33% from phase II and 25-30% from phase III.
- Of 100 drugs entering phase I, about 100 × 0.70 = 70 reach phase II.
- About 70 × 0.33 ≈ 23 reach phase III.
- About 23 × 0.25 ≈ 6 to 23 × 0.30 ≈ 7 get through phase III.
- Reading: of every 100 candidates tested in people, only about 6 or 7 go on to seek approval.
- An adverse effect in 1 of 10 000 patients: a 95% chance of seeing it at least once needs about 3 × 10 000 = 30 000 people.
- A phase III trial of 3 000 patients will rarely detect it: that is why phase IV exists.

## The phases at a glance

| Phase | Participants | Main question |
| --- | --- | --- |
| 0 (exploratory) | Very few | How does a microdose behave in the body? |
| I | About 20 to 100 people, often healthy volunteers | Is it safe, and in what dose range? |
| II | Up to several hundred patients | Does it work, and what adverse effects does it have? |
| III | About 300 to 3 000 patients or more | Is it effective against the standard or placebo? |
| IV | Thousands of patients in real practice | Do rare or late effects appear after approval? |

## Markers

- Phase I
- Phase II
- Phase III
- Phase IV

## In practice

- The rule of three: if no event was seen in n patients, the upper limit of the 95% CI for the risk is about 3/n.
- In Colombia, post-marketing pharmacovigilance is coordinated by INVIMA; health professionals can report adverse events.

## Common pitfalls

- Thinking that an approved medicine already has all its adverse effects known.
- Confusing phase IV with a bigger phase III trial: phase IV comes after approval and often includes observational studies and spontaneous reports.

## Memory aid

Is it safe? Does it work? Is it better? What happens in real life? One question per phase, I to IV.

## Test yourself

Think of the answer, then tap the question to see it.

## What is the main aim of phase I?

To assess safety and the tolerable dose range in a small group of healthy volunteers or patients.

## Which phase usually detects very rare adverse effects?

Phase IV (post-marketing), because only then is the drug used by thousands or millions of people.

## A phase III trial with 1 500 patients saw no case of an adverse effect. What is the highest plausible risk?

By the rule of three, about 3/1 500 = 0.2% (1 in 500) as the upper limit of the 95% CI.

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## Connections

## Related 4

- [Randomised controlled trial](https://durgical.com/en/epidemiology/randomised-controlled-trial/)
- [Superiority, equivalence and non-inferiority trials](https://durgical.com/en/epidemiology/superiority-equivalence-and-non-inferiority-trials/)
- [Case report and case series](https://durgical.com/en/epidemiology/case-report-and-case-series/)
- [Statistical power](https://durgical.com/en/epidemiology/statistical-power/)

[← All of epidemiology](https://durgical.com/en/epidemiology/)

Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources.

Sources: [Step 3: Clinical Research](https://www.fda.gov/patients/drug-development-process/step-3-clinical-research) (U.S. Food and Drug Administration, 2018); [Clinical trials](https://www.who.int/health-topics/clinical-trials) (World Health Organization, 2026).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/epidemiology/phases-of-clinical-trials/