Clinical trials move through phases that answer different questions about a new medicine. Phase I looks mainly at safety and how the body handles the drug, Phase II explores whether it works and at what dose, Phase III confirms benefit and safety in larger patient groups, and Phase IV continues monitoring after approval. An optional Phase 0 sometimes comes before Phase I.
Knowing what each phase is for helps you understand protocols, consent forms, monitoring plans and safety reporting. For coordinators, CRAs and pharmacovigilance staff in India and the GCC, it is foundational knowledge.
Before humans: preclinical development
Before a medicine is given to people, it goes through preclinical studies: laboratory work on its mechanism, then animal studies on pharmacology, pharmacokinetics and toxicology. Pivotal safety studies are generally conducted under Good Laboratory Practice, and ICH M3(R2) links nonclinical safety studies to the timing of clinical trials.
The sponsor then applies for permission to start human trials: an Investigational New Drug application in the US, a clinical trial application in the EU, CDSCO approval under the New Drugs and Clinical Trials Rules, 2019 in India, and the relevant national authority in the GCC. Ethics committee approval is also required.
The phases at a glance
| Phase | Main goal | Who takes part | Key questions |
|---|---|---|---|
| Phase 0 (optional) | Early human pharmacokinetic data | A very small number of volunteers or patients | Does the drug reach its target and behave as predicted? |
| Phase I | Safety, tolerability and pharmacokinetics | Small groups, often healthy volunteers; patients in fields such as oncology | Is it safe? What dose range can be tolerated? How is it absorbed and cleared? |
| Phase II | Early evidence of efficacy and dose selection | Patients with the target condition, usually larger than Phase I | Does it work? Which dose gives the best balance of benefit and risk? |
| Phase III | Confirmation of efficacy and safety | Larger patient populations, often across many sites and countries | Is the benefit confirmed against placebo or standard care? What are the less common side effects? |
| Phase IV | Post-approval safety and effectiveness | Patients in routine practice | Are there rare or long-term risks? How does it perform in the real world? |
Commonly cited approximate sizes are tens of participants in Phase I, up to a few hundred in Phase II and from several hundred to several thousand in Phase III. These are rough guides only; real numbers depend on the disease, the endpoints and the statistical design.
Phase 0 and Phase I
Phase 0 studies, sometimes called exploratory or microdosing studies, give a very small, sub-therapeutic dose to a few people to check how the drug is distributed and metabolised. They do not test benefit and are not required.
Phase I is usually the first-in-human stage. Typical designs use single and multiple ascending dose cohorts, where small groups receive increasing doses and safety data are reviewed between steps. Pharmacokinetics are measured, and close monitoring, often in a dedicated unit, is essential.
Phase II
Phase II is where the drug is first tested in patients with the target condition to look for evidence that it works. It is often split into Phase IIa (proof of concept) and Phase IIb (dose-ranging), although the labels vary between sponsors.
Phase II trials commonly use randomisation and a control group, and help choose the dose and endpoints for Phase III. Many programmes stop here because benefit is insufficient or safety is unacceptable.
Phase III
Phase III trials are usually large, randomised, controlled and often multinational. They compare the new drug with placebo or an existing treatment on a pre-specified primary endpoint, supported by secondary endpoints and safety data. Results from Phase III, together with earlier data, form the core of the marketing application.
Key design features include:
- Randomisation, so treatment groups are comparable and allocation is not influenced by investigators
- Blinding, single or double, so expectations do not bias outcomes or assessments
- Pre-specified statistical analysis, following principles in ICH E9
- Independent data monitoring committees for many large or high-risk trials
Phase IV: after approval
Phase IV covers studies and surveillance after approval, some required by the regulator, such as post-authorisation safety studies. Rare adverse reactions often appear only once many more people use a medicine, which is why pharmacovigilance signal detection matters.
Ethics, regulation and GCP across all phases
Every phase must follow Good Clinical Practice. The current international standard is ICH E6(R3), finalised by ICH in 2025, supported by ICH E8(R1) on general considerations for clinical studies. Our beginner's guide to ICH GCP E6(R3) explains the key principles.
Across all phases, participants must give valid informed consent, and ethics and regulatory approvals must be in place. In India, trials must also be registered in the Clinical Trials Registry of India before enrolment. GCC requirements vary by country, so confirm them with the national regulator.
Adaptive designs
Not every programme follows a strict sequence. Adaptive designs allow pre-planned changes based on accumulating data, such as dropping a dose arm or adjusting sample size, and seamless Phase II/III designs combine dose selection and confirmation. Regulators, including the US FDA, have published guidance on adaptive designs.
Worked example: a new antidiabetic drug
Consider a hypothetical oral drug for type 2 diabetes, with trial sites in India and the UAE.
- Preclinical: laboratory and animal studies show the compound lowers blood glucose, and GLP toxicology studies define a starting dose. The sponsor submits applications to CDSCO and the UAE authority.
- Phase I: healthy volunteers receive single and then multiple ascending doses in a clinical pharmacology unit. The team measures pharmacokinetics, records adverse events and checks for hypoglycaemia.
- Phase II: adults with type 2 diabetes are randomised to placebo or one of three doses. The primary endpoint is change in HbA1c from baseline. One dose gives most of the benefit with fewer gastrointestinal effects and is chosen for Phase III.
- Phase III: two large randomised, double-blind trials compare the drug with placebo and with an established treatment across many sites. Safety follow-up includes cardiovascular events, in line with regulatory expectations for diabetes medicines.
- Approval and Phase IV: after approval, a post-marketing study and routine pharmacovigilance monitor rare adverse reactions in wider populations, including older patients and those with kidney impairment.
At each stage, the data must justify exposing more participants.
Common mistakes
- Assuming Phase I always uses healthy volunteers; many oncology trials enrol patients.
- Treating participant numbers for each phase as fixed rules.
- Confusing efficacy in controlled trials with effectiveness in routine practice.
- Believing safety monitoring ends at approval.
- Thinking ethics approval alone is enough; regulatory authorisation is usually also needed.
Key takeaway: Each trial phase answers a different question, from first safety data to confirmed benefit and long-term risk. Understand the purpose of the phase you are working on, and you will understand why the protocol asks for what it does.
Frequently Asked Questions
What is the difference between Phase II and Phase III trials?
Phase II trials are mainly exploratory. They test the drug in patients to find early evidence of benefit and to select the dose. Phase III trials are confirmatory. They are larger, usually randomised and controlled, and designed to prove that the benefit is real and the safety profile acceptable, providing the main evidence for a marketing application.
Are healthy volunteers used in all Phase I trials?
No. Many Phase I trials do use healthy volunteers, but where a drug is expected to be toxic, such as many cancer treatments, Phase I trials usually enrol patients who may benefit. The choice depends on the drug's risk profile, its mechanism and ethical considerations, and must be justified in the protocol and approved by regulators and ethics committees.
What is a Phase 0 clinical trial?
A Phase 0 trial is an optional exploratory study in which a very small number of people receive a very low, sub-therapeutic dose of a drug. The aim is to learn how the drug is distributed and metabolised in humans before committing to full development. It does not test efficacy or establish a safe therapeutic dose.
Who approves clinical trials in India?
In India, clinical trials of new drugs need permission from the Central Drugs Standard Control Organisation (CDSCO) under the New Drugs and Clinical Trials Rules, 2019, approval from a registered ethics committee and registration in the Clinical Trials Registry of India. Requirements can change, so check the current rules and CDSCO notices before planning a study.
Next steps
If you are building a career in clinical research, our Clinical Research & GCP Essentials course covers trial conduct from protocol to close-out, and Pharmacovigilance & Drug Safety Essentials explains safety monitoring across all phases. For related reading, see the clinical research associate career guide, informed consent in clinical trials and adverse event vs serious adverse event. You can also speak to a counsellor.
Looking for structured certification?
Explore Innoryx courses or speak to a counsellor about the right program for you.