Clinical trials run in four phases, and each phase answers a different question. Phase 1 asks whether a treatment is safe and at what dose. Phase 2 asks whether it works. Phase 3 asks whether it works better than what already exists. Phase 4 watches how it performs in the real world after registration. A treatment must clear each phase before the next one can begin.
Knowing the phase tells you most of what to expect as a participant: who the study recruits, how much is already known, how closely you'll be monitored and how long you'll be involved.
The four phases at a glance
| Phase | Main question | Who takes part | Typical size | Typical duration |
|---|---|---|---|---|
| Phase 1 | Is it safe, how is it tolerated, and what dose is right? | Usually healthy volunteers; patients in some fields such as oncology | Tens of people | Days to months per participant |
| Phase 2 | Does it work in the condition, and at which dose? | Patients with the condition | Dozens to a few hundred | Months to a year or more |
| Phase 3 | Is it better than existing treatment or placebo, and is it safe at scale? | Patients with the condition | Hundreds to thousands | One to several years |
| Phase 4 | How does it perform in routine use, long term? | Patients, after the treatment is registered | Varies widely, often large | Years, sometimes ongoing |
Phase 1: is it safe, and what's the dose?
The first time a treatment is given to people. Doses start low and increase in careful steps, with intensive monitoring: frequent bloods, ECGs, vitals, and in first-in-human studies, one participant dosed ahead of the rest. Many Phase 1 studies run in purpose-built residential units.
Most recruit healthy volunteers, because studying a drug's effects is clearer in people without the disease. The exception is fields like oncology, where first studies run in patients. Phase 1 carries the most unknowns and the tightest supervision of any research stage, a trade-off covered honestly in are clinical trials safe.
Phase 2: does it work?
The first test in people who have the condition. Phase 2 looks for a signal that the treatment does what it's meant to, refines the dose, and keeps building the safety picture. These studies are still relatively small, which means a promising Phase 2 result is exactly that: promising, and not proof. Many treatments that look good here don't hold up in Phase 3.
Phase 3: is it better than what we have?
The decisive test. The new treatment is compared against the best existing treatment, or a placebo where that's appropriate, in a large randomised group across many sites. Phase 3 is designed to detect both real benefit and less common side effects that small studies miss. Its results are the core of the registration case put to the TGA.
For participants, Phase 3 usually means a known side-effect profile, ordinary clinic settings, longer involvement and the classic randomisation-and-blinding design.
Phase 4: how does it perform in the real world?
Registration isn't the end of the evidence. Phase 4 studies follow a treatment in routine use: longer horizons, broader and messier populations than trials allow, rarer side effects, and questions like how it performs alongside other medicines. If Phase 3 is the exam, Phase 4 is the reference check that never really stops.
What happens before Phase 1?
Years of laboratory and animal work, usually called preclinical research. A treatment only reaches Phase 1 after that work suggests it's plausible and safe enough to test in people, and after an ethics committee and the TGA's notification or approval process have signed off on the trial itself. Most candidate treatments never make it out of the lab, and of those that do, most don't make it through all four phases. The funnel is brutal by design.
What the phase means for you as a participant
Ask "what phase is this?" early, because the answer shapes everything else: how much is known (least in Phase 1, most in Phase 4), who qualifies, how intensive the monitoring is, and how long you're committing for. Then ask the stage-by-stage questions in what to expect when you join a clinical trial, which apply whatever the phase.
And if a term in a trial listing doesn't make sense, it's probably in the glossary.
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Related reading: Healthy volunteer trials · Will I get a placebo? · Are clinical trials safe?