Clinical Trial Phases Explained: Phase 1, 2 and 3 for Investors
Clinical trials run in three main phases. Phase 1 checks safety in a small group, Phase 2 looks for a signal that the drug works and finds the dose, and Phase 3 tests it on hundreds or thousands of patients to prove it. Only about 8 in 100 drugs that start Phase 1 end up approved, which is why each result can move a biotech stock hard.
What each clinical trial phase tests
Phase 1 is the first time a drug goes into people, usually a few dozen. The question is simple: is it safe, and what dose can people tolerate? Cancer drugs often skip healthy volunteers and go straight to patients.
Phase 2 asks whether the drug does anything useful. It runs on a larger group of patients, often a hundred or more, and tries different doses. This is where most drugs fail.
Phase 3 is the big, expensive test. Hundreds or thousands of patients, usually randomized against a placebo or the current treatment, with a primary endpoint agreed on before the trial starts. If Phase 3 works, the company files with the FDA.
Is Phase 3 the last phase? Before approval, yes. After approval there is sometimes a Phase 4: studies the FDA asks for to watch long-term safety in the real world.
How many drugs make it through each phase
The best-known numbers come from the BIO, Informa and QLS study of drug programs from 2011 to 2020. From Phase 1, only 7.9% of drugs went on to win approval. The hardest step was Phase 2: only about 29% of drugs moved from Phase 2 into Phase 3.
The odds vary a lot by disease. Cancer drugs had the lowest odds of approval from Phase 1, and blood disorders the highest. A Phase 3 drug in a disease where the biology is well understood is a very different bet from a first-in-class drug in Alzheimer's.
Why Phase 3 results move biotech stocks the most
A Phase 3 readout is often the moment that decides whether a small company has a product or not. If the company has one main drug, a Phase 3 miss can take away most of its value in a single day.
Look at Praxis Precision Medicines in October 2025. Its Phase 3 trial reported, and the stock went from $57.35 to $162.71 in one day, up 184%. The date was not a secret: the trial's expected completion date was public on ClinicalTrials.gov weeks earlier.
The other side is just as real. aTyr Pharma's Phase 3 missed its goal in September 2025, and the stock fell from $6.03 to about $1 in a day.
Does an upcoming readout make a stock a better buy?
Not on its own. In our test of 22,234 past stock-months, biotechs with a late-stage readout due within six months rose 50% or more over the next six months 13.6% of the time, almost the same as the 13.3% for all biotechs. What changed the picture was the cash. Companies that were short of money before their readout fell 30% or more 43% of the time. Companies with the cash to survive a miss and at least three funds holding fell that much 19% of the time.
In other words, the readout tells you when the stock might move. The cash and the owners tell you how bad it gets if the news is bad.
How to read a trial result as an investor
- Did it hit the primary endpoint? That is the one measure the trial was built to test. Wins on secondary endpoints matter less.
- By how much? A statistically significant result can still be too small to matter to doctors.
- What about safety? A drug that works but causes serious side effects can still struggle.
- How does it compare? A drug that is only as good as an existing cheap treatment has a hard road.
See it live: See every upcoming trial readout by phase, with the completion date, trial success score, cash and runway in the same row.
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Frequently asked questions
Phase 3 is the large trial that is meant to prove a drug works and is safe enough to approve. It usually enrolls hundreds or thousands of patients, compares the drug with a placebo or the standard treatment, and measures a primary endpoint agreed on in advance. A positive Phase 3 is normally what a company needs before filing with the FDA.
It is the last phase before approval. After a drug is approved, the FDA can ask for Phase 4 studies to watch safety and long-term effects in everyday use.
It varies a lot. Phase 1 often takes about a year, Phase 2 one to two years, and Phase 3 two to four years. Rare-disease and cancer trials can move faster; large heart or Alzheimer's trials can take longer.
In the BIO, Informa and QLS study of 2011 to 2020, about 29% of drugs moved from Phase 2 into Phase 3. Phase 2 was the hardest step in the whole process.
Because the price often already assumed success. If the result is good but not as good as hoped, or the safety data raises questions, investors sell. Celcuity is a recent example: its stock fell 26% in one day in June 2026 after full Phase 3 data, even though the trial was positive.
Educational content only. Not financial advice. Past results don't predict future ones.