What are the phases of a clinical trial?

New medicines and vaccines are tested in people in stages called phases. The phase numbers follow definitions developed by the US Food and Drug Administration, and the global registry ClinicalTrials.gov uses five of them: Early Phase 1 (formerly called Phase 0), Phase 1, Phase 2, Phase 3 and Phase 4, according to its glossary. Trials of devices or behavioral programs often have no phase at all.

Each phase answers a different question, and many drugs stop along the way. That is why the phase is the first thing to check when you read about a promising new treatment.

The phases at a glance

These are the typical sizes, lengths and goals the FDA gives for each phase of a drug's development:

• Early Phase 1 (Phase 0): small exploratory studies with very limited exposure to the drug, run to see how or whether it affects the body. They have no treatment goal.
• Phase 1: 20 to 100 healthy volunteers or people with the condition, for several months. The aim is safety and finding the right dose. About 70% of drugs move on.
• Phase 2: up to several hundred people with the condition, for several months to two years. The aim is to see whether the drug works and what side effects it has. About 33% of drugs move on.
• Phase 3: 300 to 3,000 people with the condition, for one to four years. These large “pivotal” studies test whether the drug really helps and watch for less common side effects. About 25% to 30% of drugs move on.
• Phase 4: several thousand people, after the drug has been approved, to keep monitoring safety and effectiveness in everyday use.

Why early results are not proof

Phase 1 and Phase 2 studies are designed to answer narrow questions. The FDA notes that Phase 2 studies are usually too small to show whether a drug is beneficial; their job is to collect more safety data and help design Phase 3. A striking result in a few dozen patients can shrink or disappear in a larger trial with a comparison group.

In cancer research, Phase 1 trials are run in patients who have that cancer rather than in healthy volunteers, the FDA says. That is why a cancer study can report how many tumors shrank even at Phase 1.

Reading a trial in the news

The phase tells you how far a treatment still has to go. Brazil's malaria vaccine Vivaxin, for example, was cleared in October 2026 for a Phase 1 trial in 48 healthy adults that tests safety and antibody response, not whether it prevents malaria. The Parkinson's pill solengepras, by contrast, reached Phase 3 with 341 patients, the large “pivotal” stage.

A few practical questions help when you read about a trial:

• How many people took part, and was there a comparison group that received a placebo or the standard treatment?
• What did the trial measure: safety, a lab value such as a blood marker, or how patients actually felt and functioned?
• Has the result been published in a peer-reviewed journal, or only in a company press release?
• Is the treatment available anywhere yet, and when are the next results expected?

Good News Daily states the phase and the limits in every health story. Our latest clinical trial stories are collected under clinical trials.