Drug approval has long been associated with two identical pivotal clinical trials demonstrating that not only is the drug safe and effective, but also that the trial results are reproducible. New draft guidance from the US Food and Drug Administration (FDA) provides more detail on another route: in some circumstances, one well-designed clinical trial supported by other evidence may meet the agency’s standard for approval.
Issued in June 2026, the revised draft guidance, Demonstrating Substantial Evidence of Effectiveness for Human Drug and Biological Products, applies to new drug applications, biologics license applications and certain supplements. It updates draft guidance issued in 2019 and reflects changes in the evidence available to support drug development.
More Than the Number of Trials
The FDA’s assessment considers the strength of the evidence across a development program, not simply the number of studies conducted. Trial design plays a central role. Choice of control, randomization, blinding and selection of endpoints can all affect the strength of the resulting evidence. The FDA generally favors clinical endpoints when feasible, although appropriately justified surrogate endpoints may also support traditional or accelerated approval.
Execution matters as well. Protocol adherence, data quality, participant follow-up and maintenance of blinding can influence whether results provide convincing evidence. Even a strong study design can be undermined by problems such as missing data or unblinding.
Statistical significance alone is insufficient. The FDA considers whether the treatment effect is clinically meaningful. A small difference can achieve statistical significance in a large study without necessarily representing a meaningful benefit for patients.
When One Trial May Support Approval
One focus of the revised guidance is the pathway involving a single adequate and well-controlled clinical trial plus confirmatory evidence.
The FDA says this approach will generally require either a “highly persuasive” pivotal trial or strong evidence from another source. The amount and strength of supporting evidence needed will depend in part on the quality and persuasiveness of the pivotal trial.
Strong confirmatory evidence could come from controlled studies conducted in a related disease or condition. Evidence involving other approved drugs in the same pharmacological class may also be considered under appropriate circumstances. Factors such as similarity in disease biology, mechanism of action and endpoints can determine how relevant such data are.
When strong confirmatory evidence is unavailable, early-phase clinical findings may provide support if the pivotal trial itself produces “highly persuasive” results. The FDA expects such a trial to use a design relevant to US clinical practice, have sufficient statistical power, use a clinically meaningful primary endpoint and produce findings supported by secondary endpoints and patient subgroups. Trial conduct should also include thorough follow-up with little missing data.
Looking Across the Development Program
The guidance also emphasizes the value of evidence generated before the pivotal stage. Early studies may provide information about mechanism of action, dose selection and other factors that strengthen expectations of effectiveness. Knowledge about disease biology, natural history or related therapies can add context.
Evidence across a program must also be consistent. If one well-controlled study suggests that a drug provides no benefit or causes harm, the FDA may consider that finding when assessing positive results from other trials. This approach places greater importance on evidence generated throughout development rather than treating the pivotal trial as an isolated source of support for effectiveness.
Flexibility Depends on Clinical Context
The FDA also describes circumstances in which greater regulatory flexibility may be warranted. Disease severity, unmet medical need and rarity can influence how the agency applies the effectiveness standard.
For serious or severely debilitating diseases without satisfactory treatment options, some uncertainty may be acceptable when weighed against the consequences of delaying an effective therapy. The flexibility afforded by this new guidance may also affect aspects of trial design and analysis, or the types and strength of confirmatory evidence considered.
The guidance recommends that sponsors discuss their proposed evidence strategy with the FDA early in development and no later than the end-of-Phase II meeting.
For drug developers, the revised guidance provides more detail on how evidence generated across a clinical program can contribute to an approval application. With early planning, strong trial execution and evidence suited to the clinical context, sponsors may have more than one path for demonstrating a new therapy’s effectiveness.
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