TriangleWhat changes from Phase 1 to Phase 3
The phase number tells you what question the study is built to answer, and roughly how many people are in it. It does not tell you how promising the treatment is.
This is not clinical evidence and not medical advice. It does not replace your care team. Discuss anything here with your clinician before you act on it. Product questions are on the FAQ.
New cancer treatments are tested in a sequence of studies called phases. A treatment moves on only if the current results support it. Each phase is designed around a different question. I used to read a higher number as “more proven.” That is not what it means for any one person.
Phase 1 — is it safe, and at what dose?
Safety, side effects, whether people can tolerate it, and the highest tolerable dose. Groups are small, typically around 15 to 30 people. Some Phase 1 studies also look for early signs that the treatment affects the cancer, but the phase is not built to prove that. Everything on our Phase 1 pages sits here.
Phase 2 — does it seem to work?
More people, usually 50 to 100. The question shifts to whether the treatment appears to act against the cancer — shrinking tumours, slowing growth. Safety is still tracked, including short-term side effects.
Phase 3 — is it better than what we already do?
The new treatment against the current standard, on results and on side effects. People are assigned by chance (randomization) so differences between groups are not caused by who was put where. These studies are large, from a hundred to several thousand people. The FDA uses results from Phases 1 through 3 when it decides whether to approve a treatment.
After approval, Phase 4 studies follow the treatment in general use, watching long-term safety and how it behaves across larger and more varied groups.
Placebos, and what the number cannot tell you
Placebos are rare in cancer treatment trials. Most comparative studies test the new treatment against the approved standard, not against nothing. A placebo shows up only when there is no standard treatment, or when both groups get the standard and one group also gets the study drug. If a study uses one, the consent form says so before you decide.
A higher phase means more people have received the treatment. It does not mean it is more likely to help you. Whether a given study fits depends on its eligibility criteria and on your care team’s reading of your case.
See recruiting Phase 1 trials →
Sources
- NCI: How Do Clinical Trials Work? — Phases of clinical trialsPhase 1 tests safety, side effects, tolerability, and highest tolerable dose in around 15 to 30 people; it also checks that a treatment affects the cancer.
- NCI: How Do Clinical Trials Work? — Phases of clinical trialsPhase 2 includes 50 to 100 people to see whether the treatment seems to work against the cancer, while continuing to study safety.
- NCI: How Do Clinical Trials Work? — Phases of clinical trialsPhase 3 compares the treatment with current standard therapy using random assignment, in 100 to several thousand people; FDA uses Phase 1–3 results in approval decisions.
- NCI: How Do Clinical Trials Work? — Phases of clinical trialsPhase 4 looks at long-term safety and effectiveness after FDA approval in large, diverse populations.
- NCI: How Do Clinical Trials Work? — Placebos in clinical trialsPlacebos are rarely used in cancer treatment trials; they are used when no standard exists or alongside standard treatment, and the consent form discloses their use.
- FDA: The Drug Development Process — Step 3: Clinical ResearchPhase 1 studies are primarily about safety and dosing in people.