Every prescription medicine you've ever taken went through the same four-phase gauntlet before reaching a pharmacy shelf. Understanding those phases helps you make sense of what a study is actually asking of you.
Phase 1: Is it safe in humans?
Who enrolls: Usually 20–100 people. For most drugs these are healthy volunteers. For drugs targeting serious conditions like cancer, Phase 1 enrolls patients who have exhausted standard treatments.
What's being tested: Basic safety, how the body processes the drug, and what dose range is tolerable. Phase 1 studies frequently start at very low doses and escalate carefully.
Time commitment: Often the most intensive. Many Phase 1 studies are conducted at inpatient units where participants stay for days or weeks so the team can monitor vitals around the clock.
Compensation: The highest of any phase, precisely because of the time and inconvenience — typical payouts run $2,000–$8,000 for a multi-week stay.
Phase 2: Does it work, and what's the right dose?
Who enrolls: A few hundred people who have the condition the drug is meant to treat.
What's being tested: Preliminary efficacy (does the drug do what it's supposed to?) and dose optimization. Side effects are watched closely because this is the first time the drug is used in patients rather than healthy volunteers.
Time commitment: Moderate. Usually outpatient visits every few weeks for several months.
Compensation: Modest per-visit payments plus travel, though the bigger draw is access to a potentially better treatment.
Phase 3: Does it work better than what we already have?
Who enrolls: Hundreds to several thousand people with the condition, at many clinics across the country and often internationally.
What's being tested: Efficacy compared against either a placebo or the current standard-of-care treatment. This is the phase that generates the evidence the FDA reviews for approval.
Time commitment: Often the longest — one to four years of periodic visits. You may or may not know whether you're receiving the study drug or the comparator; most Phase 3 trials are "blinded" for scientific integrity.
Compensation: Per-visit stipends plus travel. Many participants stay in these trials for the free access to the study drug and specialist care.
Phase 4: What happens once real-world doctors prescribe it?
Who enrolls: Thousands to hundreds of thousands of people, often anyone whose doctor is already prescribing the drug.
What's being tested: Long-term safety, rare side effects that only show up when the drug is used in a very large population, and effectiveness in real-world use rather than the tightly controlled conditions of Phase 3.
Time commitment: Usually minimal — often just periodic surveys or letting the study team access your medical records with permission.
Compensation: Small or none. The value here is contributing to safety data that protects future patients.
Which phase should you consider?
If you want the most controlled environment and the highest pay, Phase 1 healthy-volunteer studies are worth exploring — but they're a real time commitment.
If you have a condition and you're looking for potential access to new treatment, Phase 2 and Phase 3 studies are usually the right fit. Phase 3 studies are less experimental — a lot is already known about the drug by that point.
If you just want to help science with minimal disruption, Phase 4 observational studies and registries are a great low-effort option.
The safety picture
Every phase has an independent ethics board (an IRB) that reviews the study before it starts and continues to monitor for safety signals. Serious adverse events must be reported to the FDA. Studies get paused or stopped when safety concerns arise — that's the system working.
None of this is a replacement for a conversation with your doctor about whether a specific trial makes sense for you. But now, when a coordinator tells you a study is "Phase 2" or "Phase 3," you'll know what that actually means.
Ready to look at studies by phase and condition? [Start your search here](/#find-trials).