How Much Evidence Proves a Drug Works? Two Trials, One Trial, or One Plus Confirmatory Evidence

United States law does not ask whether a drug might work. It asks for "substantial evidence" of effectiveness, a phrase written into the Federal Food, Drug, and Cosmetic Act. For decades the Food and Drug Administration read that standard to require two adequate and well-controlled clinical trials that independently point the same direction. A 1997 statutory amendment made explicit that one such trial plus confirmatory evidence can also qualify. In draft guidance issued in 2026, the agency has begun to treat that single-trial-plus-confirmatory-evidence route as the primary approach rather than the exception.

This article explains what that standard means, why the number of trials matters, and how the evidentiary bar is being reframed. It is educational and not medical advice.

What "substantial evidence" actually requires

The operative language sits in section 505(d) of the FD&C Act. Congress defined substantial evidence as evidence consisting of "adequate and well-controlled investigations" from which experts could fairly conclude the drug will have its claimed effect. Two ideas are packed into that sentence. The first is a quality requirement: each study must be adequate and well-controlled, meaning it has a clear objective, a valid comparison (often a placebo or active control), sound methods to reduce bias such as randomization and blinding, and a prespecified analysis. The second is a quantity question hiding inside a single plural word. Congress wrote "investigations," and since the early 1960s the FDA has generally interpreted that plural to mean two or more.

The rationale for two was never bureaucratic caution for its own sake. A single positive trial can mislead. Chance, an unrepresentative population, an unusual site effect, or subtle bias can all produce a result that does not replicate. Independent substantiation, in the classic framing, guards against a false conclusion that a drug works when it does not. The FDA's guidance on this subject, "Demonstrating Substantial Evidence of Effectiveness for Human Drug and Biological Products," describes both the quality features of an adequate and well-controlled study and the circumstances under which the quantity of evidence can flex.

FDAMA section 115 and the one-trial pathway

The rigid reading of "investigations" as always two softened by statute. In the Food and Drug Administration Modernization Act of 1997 (Public Law 105-115), Congress added language to section 505(d) clarifying that the agency may consider "data from one adequate and well-controlled clinical investigation and confirmatory evidence" to constitute substantial evidence, if the FDA judges those data sufficient. That provision is widely known by its section number: FDAMA 115.

The point of FDAMA 115 was not to lower the effectiveness standard but to recognize that a single, unusually persuasive trial supported by independent evidence can carry the same conviction as two standalone studies. The statute does not define confirmatory evidence, which left the practical question open for years: what counts as enough to confirm a single trial?

What confirmatory evidence looks like

In 2023 the FDA published draft guidance titled "Demonstrating Substantial Evidence of Effectiveness Based on One Adequate and Well-Controlled Clinical Investigation and Confirmatory Evidence," its most detailed attempt to answer that question. The draft describes several categories of evidence that can support a single pivotal trial rather than substitute for it. These include data from a related, adequate and well-controlled trial; evidence supporting a closely related indication for the same drug; results from other approved drugs in the same pharmacologic class; mechanistic and pharmacologic understanding of how the drug acts on the disease; and natural history or registry data that clarify the expected course of the illness.

The logic is that confirmation can come from a source other than a second identical trial. If a drug's mechanism is well understood, if the disease's untreated trajectory is well characterized, and if related agents behave consistently, a single rigorous trial may sit inside a web of mutually reinforcing evidence. The FDA has stressed that confirmatory evidence must be scientifically relevant and reliable, not merely suggestive.

The 2026 reframing

The framing shifted further in 2026. In February, senior FDA leadership argued in a New England Journal of Medicine article that one adequate and well-controlled study combined with confirmatory evidence should serve as the ordinary basis for marketing authorization, reversing the presumption that two trials are the norm and single-trial approval the exception. In June 2026, as part of a broader initiative the agency described as Operation TrialBlazer, the FDA issued a revised draft guidance, "Demonstrating Substantial Evidence of Effectiveness for Human Drug and Biological Products," published in the Federal Register on June 24, 2026 with a comment period running into September. When finalized, that guidance is intended to replace the longstanding 1998 guidance. The revised draft centers a single adequate and well-controlled investigation with confirmatory evidence as the primary route and lists sources such as related trial data, data from drugs in the same pharmacologic class, natural history and registry data, and external information on disease pathophysiology.

One point deserves emphasis. These are draft guidances, not law and not final policy. Guidance documents describe the agency's current thinking; the statutory standard in section 505(d) is unchanged, and a draft can move substantially before it is finalized. The trade-off at the center of the debate is real and long-standing. Fewer required trials can bring effective treatments to patients sooner and reduce cost, especially in rare diseases where a second large trial may be impractical. The same flexibility places more weight on the quality of a single trial and on the strength of confirmatory evidence, raising the stakes if that supporting evidence is weaker than it appears. How that balance is struck, and how consistently, is the substance of the current comment period.