Clinical trials run in four phases. Phase 1 tests safety and dosage, usually in healthy volunteers, and pays the most. Phase 2 tests whether the treatment works in people with the condition. Phase 3 compares it against the current standard in large groups, and Phase 4 monitors it after it reaches the market. A drug must clear each phase before moving to the next.
Understanding the phases is the fastest way to make sense of trial listings, because the phase tells you almost everything: who the study wants, how demanding it will be, roughly what it pays, and what is known about the compound so far. Here is each phase in plain English, followed by a side-by-side table.
Phase 1: safety and dosage
Phase 1 is the first time a compound is given to humans. The goal is not to treat anything — it is to learn how the drug behaves in the body: how fast it is absorbed, how it is metabolized, what side effects appear, and what dose range is tolerable. Groups are small, dosing starts low and escalates carefully, and participants are monitored intensively, often during overnight stays in a dedicated research unit.
Because no therapeutic benefit is expected, Phase 1 mostly enrolls healthy volunteers, and compensation is the highest of any phase — the median Phase 1 study pays around $3,070. This is the phase where the paid-volunteer world concentrates.
Phase 2: does it work?
Once a safe dose range is established, Phase 2 gives the drug to people who actually have the target condition, typically a few dozen to a few hundred of them. The question shifts from is it safe enough to does it do anything: researchers measure the treatment effect while continuing to track side effects and refine dosing.
Phase 2 studies are usually outpatient, with scheduled clinic visits over weeks or months. Compensation is moderate — often structured per visit — because the time burden is lighter than Phase 1 confinement and participants may benefit from the treatment itself. Many Phase 2 trials use blinding and placebo controls; our guide to double-blind studies explains what that means for you.
Phase 3: comparison at scale
Phase 3 is the decisive test. The new treatment is compared against the current standard of care, or against placebo where no standard exists, in large groups — hundreds to thousands of participants across many sites, often internationally. These are the randomized controlled trials that regulators weigh most heavily when deciding on approval.
For participants, Phase 3 means longer timelines but usually routine outpatient visits. Compensation per hour of involvement is lower than early phases, though visit payments and travel reimbursements are standard. Because sites need large numbers, Phase 3 trials recruit broadly, and big metro areas like Philadelphia and Houston typically have dozens open at once.
Phase 4: after approval
Phase 4 studies happen after a treatment is already on the market. Regulators or sponsors keep studying it to catch rare side effects that only surface at population scale, to test long-term outcomes, or to explore use in new groups. Some Phase 4 work is observational — tracking people already taking the drug through registries and follow-up visits — rather than assigning treatment.
The burden on participants is usually light, and pay reflects that: short observational studies commonly pay in the $25 to $100 per visit range. Phase 4 can be a low-commitment entry point if you want to experience research participation before considering anything more intensive, with schedules that rarely disrupt work or family routines.
Join the free email alert — we watch the NIH registry weekly so you don't have to.
Get trial alertsThe four phases side by side
| Phase | Main question | Typical size | Typical duration | Who enrolls | Relative pay |
|---|---|---|---|---|---|
| Phase 1 | Is it safe? What dose? | Small groups (dozens) | Days to months, often inpatient | Mostly healthy volunteers | Highest |
| Phase 2 | Does it work? | Dozens to hundreds | Several months | Patients with the condition | Moderate |
| Phase 3 | Is it better than the standard? | Hundreds to thousands | One to several years | Patients, broad criteria | Lower per visit |
| Phase 4 | What happens at scale, long term? | Very large | Ongoing | People using the approved treatment | Lowest |
Every phase, at every legitimate site, shares the same guardrails: registration on ClinicalTrials.gov, IRB oversight, written informed consent, and the right to withdraw at any time. For what those protections mean in practice, read are clinical trials safe.
Where a healthy volunteer fits
If you have no diagnosis and want paid participation, your realistic entry points are Phase 1 drug studies, vaccine trials at any phase, and observational research recruiting from the general population. Phase 2 and 3 drug trials will screen you out by design, since their inclusion criteria require the condition being studied. Reading a listing, the phase line therefore answers your first question — is this study even for me — before you spend a minute on the rest of the criteria. Compensation follows the same line: the earlier the phase, the heavier your schedule and the larger the payment.