Research
What changes between phase 1 and phase 3
The phases of a drug trial are usually described by how many people are in them. The regulation describes something more useful: what each stage is asking, and how much a company owes the regulator at each one.
The phases are stages of a permission, not sizes of a study
Phase labels get used on product pages as a scale of credibility. A phase 2 sounds better than a phase 1, and a phase 3 sounds close to approved.
The regulation that defines them is doing something else. It describes what each stage is for, and the rest of the part attaches different obligations to each.
What actually changes between the stages is the question being asked and the amount of detail a company has to hand over. Reading it that way makes a phase label far more informative than a ranking.
What the rule says the first stage is
The first phase includes the initial introduction of an investigational new drug into humans. Those studies are typically closely monitored, and may be conducted in patients or in normal volunteer subjects.
Their purpose is stated in three parts. To determine the metabolism and pharmacologic actions of the drug in humans, to find the side effects associated with increasing doses, and, if possible, to gain early evidence on effectiveness.
There is also a forward-looking requirement. Enough information about pharmacokinetics and pharmacological effects should be obtained to permit the design of well-controlled, scientifically valid second-phase studies.
On size, the rule declines to be precise. The total number of subjects and patients varies with the drug, but is generally in the range of 20 to 80.
Two published numbers, and how to hold them
The agency's public explainer of clinical research gives a figure too, and it gives two.
Its summary table lists study participants for the first phase as 20 to 100 healthy volunteers or people with the disease or condition. Two sentences into the narrative below, the same page says that in most cases 20 to 80 participate.
Both figures are published by the same body on the same page, and the lower one matches the regulation.
The useful lesson is smaller than a contradiction and more practical. A number lifted from a summary box may not be the number the surrounding text supports, and neither is a limit.
The paragraph that says a first-phase study need not be a treatment study
The definition has a second paragraph that rarely gets quoted, and it changes how a first-phase citation should be read.
First-phase studies also include studies of drug metabolism, structure-activity relationships, and mechanism of action in humans.
They further include studies in which investigational drugs are used as research tools to explore biological phenomena or disease processes.
That is a study where the drug is the instrument, not the candidate treatment. The point of it may be to learn about the body rather than about the product.
So a page that cites a first-phase study as evidence a compound works has not yet established what the study was asking. That has to be read out of the study itself.
The second stage, and the word controlled
The second phase is where effectiveness first becomes the question, and the rule is specific about the design.
It includes the controlled clinical studies conducted to evaluate the effectiveness of the drug for a particular indication in patients with the disease or condition under study.
It has a second job at the same time: to determine the common short-term side effects and risks associated with the drug.
Size and rigor go together here. These studies are typically well controlled, closely monitored, and conducted in a relatively small number of patients, usually no more than several hundred.
Two words in that sentence do most of the work. Controlled, meaning there is a comparison. And indication, meaning the finding attaches to a specific use rather than to the compound at large.
The third stage, which is not only large trials
The third phase is commonly described as the big one, and the rule describes it as expanded controlled and uncontrolled trials.
Uncontrolled belongs in that sentence. Not every study at this stage has a comparison group, which is worth knowing before treating a third-phase citation as automatically stronger.
These trials are performed after preliminary evidence suggesting effectiveness has been obtained. They gather the additional information about effectiveness and safety needed to evaluate the overall benefit-risk relationship.
And they have a second purpose that explains their size. They provide an adequate basis for physician labeling, meaning the instructions a prescriber will eventually read.
The rule gives a range rather than a threshold: usually from several hundred to several thousand subjects.
They are not a queue
Two sentences in the same regulation undercut the idea of phases as rungs on a ladder.
An application may be submitted for one or more phases of an investigation, so a single filing can cover more than one stage.
And although in general the phases are conducted sequentially, they may overlap.
A program can therefore be running two stages at once. A phase label on one study says what that study was designed to do, not where the whole program has arrived.
What a company owes at each stage: the protocol
The clearest expression of what changes is in the filing rules, which scale the paperwork to the stage.
First-phase protocols may be less detailed and more flexible. They are meant to outline the investigation, and to specify in detail only those elements critical to safety, such as necessary monitoring of vital signs and blood chemistries.
Second and third phase protocols are the opposite. Detailed protocols describing all aspects of the study are expected.
They also have to anticipate their own drift. Where the sponsor expects some deviation from the design may become necessary, alternatives or contingencies must be built into the protocol at the outset.
What a company owes at each stage: the manufacturing detail
The chemistry and manufacturing section scales the same way, and the rule explains why.
The amount of information needed varies with the phase, the proposed duration, the dosage form, and how much is otherwise available.
Early on, the emphasis should generally be on identification and control of the raw materials and the new drug substance.
One sentence there is easy to misread as permission. Final specifications for the drug substance and drug product are not expected until the end of the investigational process.
That is a statement about a supervised process arriving at a specification, not about a product being allowed to lack one. Stability data are required in all phases, and the requirement scales with the length of the study rather than disappearing.
What counts as a change big enough to file
The threshold for reporting a mid-study change is set by phase, and it is the sharpest illustration of the difference between the stages.
For a first-phase protocol, an amendment is required for any change that significantly affects the safety of subjects.
For a second or third phase protocol, the trigger is wider. A change significantly affecting the safety of subjects, the scope of the investigation, or the scientific quality of the study.
Scope and scientific quality are added because, by then, the study is producing the evidence a marketing decision will rest on.
Smaller first-phase changes are not invisible either. Modifications that do not affect critical safety assessments are reported to the agency in the annual report.
Reading a phase label on a product page
Start with what the study was asking, not with the number. A first-phase study may have been a mechanistic probe using the drug as a research tool.
Check whether there was a comparison. Controlled is written into the second-phase definition and is optional at the third.
Check the indication. Effectiveness at the second stage attaches to a particular use in patients with a particular condition.
And treat the label as a description of one study rather than a status for the program, since the rule expects the phases to overlap.
Key takeaways
- The regulation defines the phases by the question each one asks, and the surrounding rules attach different obligations to each.
- A first-phase study can be a mechanistic probe, including one that uses the drug as a research tool to explore biological phenomena.
- The rule puts first-phase enrollment generally in the range of 20 to 80; the agency's public explainer prints both 20 to 100 and 20 to 80 on one page.
- Second-phase studies are the controlled ones evaluating effectiveness for a particular indication, usually in no more than several hundred patients.
- Third-phase work includes uncontrolled trials as well as controlled ones, and exists partly to support physician labeling.
- Phases may overlap, and one application may cover more than one, so a label describes a study rather than a program.
- Protocol and manufacturing detail scale by phase, and the threshold for filing a mid-study change widens after the first phase.
Frequently asked questions
How many people are in a first-phase study?
The regulation says the total varies with the drug but is generally in the range of 20 to 80. The agency's public explainer of clinical research prints 20 to 100 in its summary table and, in the narrative below it, says that in most cases 20 to 80 participate. Both figures come from the same page, and the lower one matches the rule.
Is a first-phase study always testing whether a drug works?
No. Its stated purposes are to determine metabolism and pharmacologic actions in humans, to find the side effects associated with increasing doses, and, if possible, to gain early evidence on effectiveness. The definition also covers studies of drug metabolism, structure-activity relationships and mechanism of action, and studies where investigational drugs are used as research tools to explore biological phenomena or disease processes.
Does a third-phase trial always have a control group?
Not necessarily. The regulation describes this stage as expanded controlled and uncontrolled trials. They are run after preliminary evidence suggesting effectiveness has been obtained, to gather the additional effectiveness and safety information needed to weigh overall benefit against risk and to provide an adequate basis for physician labeling.
Do the phases happen one after another?
Usually, but not necessarily. The rule says that although in general the phases are conducted sequentially, they may overlap. It also allows a single application to be submitted for one or more phases. A phase label describes an individual study's design, not the stage a whole program has reached.
Does less paperwork early mean the standards are lower?
It means the requirements scale with what is at stake and what is knowable. Early protocols may be less detailed but must specify the elements critical to safety. Manufacturing information starts with raw material and drug substance control, and stability data are required in all phases, with the supporting data scaled to the length of the proposed study.
When does a change to a study have to be filed?
For a first-phase protocol, when the change significantly affects the safety of subjects. For a second or third phase protocol, when it significantly affects the safety of subjects, the scope of the investigation, or the scientific quality of the study. Smaller first-phase modifications that do not affect critical safety assessments are reported in the annual report instead.
Sources
Each document below is named as it names itself, with the date printed on that document rather than the day it was read.
- 21 CFR 312.21 — Phases of an investigation — Office of the Federal Register, Electronic Code of Federal Regulations, August 2026
- 21 CFR 312.23 — IND content and format, whose protocol and manufacturing items scale by phase — Office of the Federal Register, Electronic Code of Federal Regulations, August 2026
- 21 CFR 312.30 — Protocol amendments — Office of the Federal Register, Electronic Code of Federal Regulations, August 2026
- Step 3: Clinical Research — U.S. Food and Drug Administration, January 2018