Phase 3 Clinical Trial

Key facts

Defined in

21 CFR 312.21(c)

Typical size (FDA regulation)

Several hundred to several thousand subjects

Typical design

Randomized, controlled, often blinded, often multicenter

Evidence standard

Substantial evidence of effectiveness, FD&C Act Section 505(d)

Leads to

New Drug Application (NDA) or Biologics License Application (BLA)

What is a Phase 3 clinical trial?

Phase 3 trials begin after Phase 2 has produced preliminary evidence that a drug works. FDA regulations describe them as expanded controlled and uncontrolled trials intended to gather the additional information about effectiveness and safety needed to evaluate the overall benefit-risk relationship of the drug and to provide an adequate basis for physician labeling.

Phase 3 trials that form the main basis of an application are often called pivotal trials. Their results, together with data from earlier phases, go into a New Drug Application (NDA) for a drug or a Biologics License Application (BLA) for a biologic.

How Phase 3 trials are designed

Phase 3 trials are built to meet the substantial evidence standard, which FDA regulations tie to adequate and well-controlled investigations (21 CFR 314.126). Typical features include:

• Randomization to the investigational drug or a control, such as placebo, an active comparator or standard of care.
• Blinding of participants and investigators where feasible, to limit bias in how outcomes are assessed.
• A prespecified primary endpoint and statistical analysis plan, often discussed with FDA at an End-of-Phase 2 meeting.
• Many sites, frequently across several countries, to enroll enough patients and reflect the intended population.

One pivotal trial or two?

FDA has historically expected two adequate and well-controlled trials. Since the Food and Drug Administration Modernization Act of 1997, the FD&C Act also allows FDA to rely on one adequate and well-controlled trial plus confirmatory evidence, and some approvals, especially in rare diseases, rest on that basis.

Accelerated approval can rest on a surrogate endpoint, with a confirmatory trial finished after approval, so a Phase 3 program is not always complete at the time of launch.

Why Phase 3 trials matter

Phase 3 is usually the largest and costliest stage of drug development and the last step before an approval filing, so it anchors forecasts across the industry.

• Topline Phase 3 results are among the most market-moving events in biopharma.
• Primary completion dates on ClinicalTrials.gov help estimate when an NDA or BLA filing, and a launch, could follow.
• Comparators and endpoints preview how a product will be positioned against competitors and what evidence payers will see.
• Phase 3 site lists show which institutions and investigators a sponsor relies on, which helps commercial teams plan launch engagement.