Clinical trial phases are the steps a new drug or device goes through in people before and after approval. Phase I tests safety and dosage in 20 to 100 people. Phase II checks whether it works, and its side effects, in up to several hundred patients. Phase III confirms the benefit and watches for side effects in 300 to 3,000 patients over 1 to 4 years. Phase IV follows the medicine after approval. Only about 7.9% of drugs that enter Phase I are finally approved (BIO, 2011–2020).
Key takeaways
- Phase 0 (optional): micro-doses in a few people to see how a drug behaves.
- Phase I: safety and dose in 20–100 people; about 70% move on (FDA).
- Phase II: does it work? Up to several hundred patients; about 33% move on (FDA).
- Phase III: 300–3,000 patients for 1–4 years; the evidence regulators approve on.
- Phase IV: safety monitoring after the medicine is on the market.
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What are clinical trial phases?
A clinical trial tests a medicine, vaccine or device in people. Trials run in steps called phases. Each phase answers a different question — is it safe, does it work, is it better than what we have, and is it still safe in everyday use? A drug moves to the next phase only if the results are good enough. New to the field? Start with what is clinical research.
Main question
- Phase 0
- How does the drug behave in the body?
- Phase I
- Is it safe? What dose?
- Phase II
- Does it work? Side effects?
- Phase III
- Is it better or as good, and safe?
- Phase IV
- Is it safe in everyday use?
Participants
- Phase 0
- A few people
- Phase I
- 20 to 100
- Phase II
- Up to several hundred
- Phase III
- 300 to 3,000
- Phase IV
- Several thousand
Length
- Phase 0
- Short
- Phase I
- Several months
- Phase II
- Several months to 2 years
- Phase III
- 1 to 4 years
- Phase IV
- Ongoing after approval
Move to next phase
- Phase 0
- Optional, exploratory
- Phase I
- About 70%
- Phase II
- About 33%
- Phase III
- About 25–30%
- Phase IV
- —
Source: US FDA, Step 3: Clinical Research. Phase 0 is not an FDA phase in that table; it is an optional exploratory study.
Phase 0: exploratory first-in-human studies
Phase 0 studies give a very small, non-therapeutic dose to a handful of people. They check how the drug is absorbed and where it goes in the body, so a company can decide early whether to continue. They are optional and are not used to judge safety or benefit.
Phase I clinical trials: safety and dosage
Phase I is the first full test in people. A small group — usually 20 to 100 healthy volunteers, or patients for drugs such as cancer medicines — receives the drug at slowly rising doses. Researchers study:
- Safety and tolerability — what side effects appear, and at what dose
- Pharmacokinetics — how the body absorbs, breaks down and removes the drug
- Pharmacodynamics — what the drug does in the body
- The safe dose range to use in Phase II
Healthy volunteers are usually paid for their time. About 70% of drugs move from Phase I to Phase II, according to the FDA.
Phase II: does the treatment work?
Phase II tests the drug in up to several hundred patients who have the condition. It runs from several months to about 2 years. Most Phase II studies are randomised: patients are put by chance into a group that gets the new drug or a control group that gets standard treatment or a placebo. Many are blinded, so patients — and often doctors — do not know who gets what, which reduces bias.
This is the hardest step. BIO’s analysis of 2011–2020 data found that only 28.9% of drugs passed from Phase II to Phase III — usually because they did not work well enough.
Phase III: confirming benefit and safety
Phase III trials involve 300 to 3,000 patients, often at many hospitals in several countries, and last 1 to 4 years. They compare the new drug with the current standard treatment or a placebo. Because more people take part for longer, Phase III finds side effects that smaller trials miss.
If the results are good, the company files for approval — a New Drug Application (NDA) or Biologics License Application (BLA) in the US, or an application to the CDSCO in India. Most Indian trial jobs are on Phase III studies: they are the largest share of recruiting trials in India.
Phase IV: post-marketing surveillance
Phase IV studies run after a medicine is approved and on sale. They follow thousands of people in everyday use to find rare or long-term side effects, compare the medicine with others, and study its long-term benefit and cost. Results can lead to new warnings, limits on use or, rarely, withdrawal from the market. This is the main work of pharmacovigilance.
How many drugs make it through all clinical trial phases?
Very few. The largest recent analysis, by BIO, Informa Pharma Intelligence and QLS, tracked FDA development programmes from 2011 to 2020:
Success rate
- Phase I → Phase II
- 52.0%
- Phase II → Phase III
- 28.9%
- Phase III → NDA/BLA filing
- 57.8%
- NDA/BLA → FDA approval
- 90.6%
- Overall: Phase I → approval
- 7.9%
Source: BIO, Clinical Development Success Rates 2011–2020. The FDA’s simpler patient guide gives rounder figures (about 70%, 33% and 25–30%), based on older data.
Clinical trial phases in India
In India, clinical trials follow the New Drugs and Clinical Trials Rules, 2019. A trial needs permission from the Central Drugs Standard Control Organisation (CDSCO), headed by the Drugs Controller General of India, and approval from an ethics committee registered under the rules. New drugs, new formulations and new fixed-dose combinations all need clinical data before they can be sold in India.
Recruiting trials
- Phase 1
- 20
- Phase 2
- 72
- Phase 3
- 227
- Phase 4
- 29
Combined-phase studies (for example Phase 2/3) count in each phase. Source: ClinicalTrials.gov. See clinical research in India and current clinical trial trends for more data.
Who works on each phase?
CRC and CRA
Run and monitor Phase I–III trials at hospitals and research sites. See the CRA training program.
Clinical data management
Build the trial database and clean the data in every phase. See clinical data management training.
Drug safety
Report side effects during trials and in Phase IV. See pharmacovigilance training.
Clinical SAS
Analyse trial data and prepare results for regulators. See the Clinical SAS course.
FAQs
What are the 4 phases of clinical trials?
Phase I tests safety and dosage in 20 to 100 people. Phase II tests whether the drug works, and its side effects, in up to several hundred patients. Phase III confirms the benefit in 300 to 3,000 patients. Phase IV monitors the medicine after approval.
What is Phase 0?
An optional, exploratory study that gives a very small dose to a few people to see how the drug behaves in the body before full Phase I testing.
Which phase do most drugs fail?
Phase II. Only 28.9% of drugs moved from Phase II to Phase III in BIO’s 2011–2020 analysis, usually because they did not work well enough.
How many drugs that start trials get approved?
About 7.9% of drugs that enter Phase I are finally approved by the FDA, according to BIO’s 2011–2020 analysis.
How long do clinical trials take?
Phase I takes several months, Phase II several months to 2 years, and Phase III 1 to 4 years. Phase IV continues after approval. In total it often takes many years.
What is a randomised controlled trial?
A trial in which participants are put by chance into a group that gets the new treatment or a control group, so the results can be compared fairly.
Are clinical trial participants paid?
Healthy volunteers in Phase I are usually paid. In later phases, patients are often reimbursed for costs such as travel.
Who approves clinical trials in India?
The CDSCO, headed by the Drugs Controller General of India, under the New Drugs and Clinical Trials Rules, 2019, together with a registered ethics committee.