A Phase 1 clinical trial is the first time an experimental drug or treatment is tested in humans. Its primary goal is safety: researchers want to find out whether a new compound is safe enough to keep testing, what side effects it causes, and how the human body processes it. These trials are small, typically involving 20 to 80 participants, and they represent the earliest step in a long process that determines whether a drug eventually reaches the market.
What Happens in a Phase 1 Trial
Before any drug can be tested in people in the United States, its developer must submit an Investigational New Drug (IND) application to the FDA. This filing includes animal toxicology data, details about how the drug is manufactured, a plan for the study, and any prior human experience with the compound. The FDA reviews this package and, if it doesn’t object within 30 days, the trial can begin.
Once the trial starts, researchers aren’t trying to prove the drug works. They’re asking more fundamental questions: Is this safe for humans? How much can a person tolerate? How does the body absorb, break down, and eliminate the drug? To answer these, the research team collects frequent blood and urine samples, monitors organ function through lab tests, and tracks every symptom participants report.
How Dosing Works
Phase 1 trials don’t give everyone the same dose. Instead, researchers use a careful dose escalation process, starting at a level considered safe based on animal studies and gradually increasing it. The most common approach is called the 3+3 design: three participants receive a low starting dose, and if none of them experience a serious side effect, the next group of three receives a higher dose.
If one person in a group of three does have a significant side effect (called a dose-limiting toxicity), three more people are treated at that same dose to see if it was a fluke or a pattern. Escalation continues until at least two out of three to six participants experience a serious side effect at a given dose level. The dose just below that threshold becomes the recommended dose for the next phase of testing. Early dose increases tend to be large (sometimes doubling), then get progressively smaller as researchers approach the upper boundary of tolerability.
Some newer trials use statistical models instead of rigid rules. These designs use toxicity data from every enrolled patient to build a mathematical dose-response curve, allowing researchers to zero in on the ideal dose more efficiently.
Who Participates
Most Phase 1 trials enroll healthy volunteers. These are people without the disease the drug is meant to treat, and they participate specifically to help researchers understand the drug’s basic safety profile and how it moves through the body.
Cancer drugs are the major exception. Because chemotherapy and many targeted cancer treatments are inherently toxic to healthy cells, it would be unethical to give them to healthy people. Phase 1 cancer trials instead enroll patients who have already exhausted standard treatment options. For these participants, the trial serves a dual purpose: it tests safety while also giving them access to a treatment that might help. Historically, tumor response rates in Phase 1 cancer trials have been low, around 4% to 5%, but those numbers have been improving as drug development becomes more precise. The risk of death caused by the experimental treatment itself has remained very low, under 1%.
Safety Monitoring During the Trial
Participants in Phase 1 trials are monitored closely, often spending hours or even days at the trial site after receiving a dose. Researchers track vital signs, run blood panels to check liver and kidney function, and document every adverse event, no matter how minor.
For trials that pose higher risk, an independent group called a Data Safety Monitoring Board (DSMB) may oversee the study. These boards are made up of outside experts with no financial stake in the drug’s success. They periodically review the accumulating safety data and can recommend that the trial continue as planned, be modified, temporarily paused, or stopped entirely if participants are being harmed. DSMBs are more commonly associated with later-phase trials, but they’re used in Phase 1 when the intervention carries meaningful risk.
How Phase 1 Fits Into Drug Development
Drug development follows a structured sequence. Before Phase 1, a compound goes through years of laboratory and animal testing. Phase 1 is the first human checkpoint, and it answers the most basic question: can people tolerate this drug at all?
If a drug clears Phase 1, it moves to Phase 2, where researchers test it in a larger group of people (typically a few hundred) who actually have the disease. Phase 2 starts to examine whether the drug works, not just whether it’s safe. Phase 3 trials are larger still, often involving thousands of participants, and they generate the evidence needed for FDA approval.
The entire journey from Phase 1 to an approved drug takes years, and most compounds don’t make it. A Phase 1 trial itself typically lasts several months, though the exact timeline depends on how many dose levels need to be tested and how quickly participants can be enrolled. The small size of these studies, usually 20 to 80 people, keeps them relatively short compared to later phases, but the data they produce shapes every decision that follows.