Clinical Trial Phases Explained: What Happens in Phases 0–4
Clinical trial phases are the structured stages a new therapy, medication, or drug moves through before becoming part of routine health care. These phases help researchers understand safety, efficacy, dosing, and long-term health outcomes. If you are considering enrolling in a clinical trial, you can explore current opportunities on our Studies page to see which trials are open now.
Each stage of the clinical trials process answers a different research question—from early safety testing in a phase 1 study to large-scale comparisons in a phase III clinical trial. Most community-based clinical research occurs during phase 2 and phase 3, and many medical research studies in Tampa take place right here at Santos Research Center.
This guide explains the clinical trial phases explained clearly, how study designs work, and what patients can expect at each stage of drug development.
Quick Overview of Clinical Trial Phases (0–4)
The phases of clinical research follow a predictable sequence. Each phase serves a specific role in the phases of drug development and FDA clinical trial phases.
| Phase | Main Question | Typical Participants | What Researchers Measure |
|---|---|---|---|
| Phase 0 | How does the drug behave in the body? | Very small group | Absorption, metabolism, clearance |
| Phase 1 | Is it safe? What dose is best? | Healthy volunteer paid clinical trials or condition-specific patients | Side effects, safety signals |
| Phase 2 | Does it help the disease? | People with the disease | Symptom improvement, biomarker changes |
| Phase 3 | How does it compare to standard treatment? | Large multi-site patient groups | Effectiveness, safety, quality of life |
| Phase 4 | How does it perform in real-world use? | Patients prescribed the medication | Long-term risks, rare effects |
Together, these clinical trial phases 1–4 protect patients and ensure new treatments are supported by reliable medical research.
Phase 0 — Early Exploratory Studies
Purpose: Understand how a potential drug behaves in the human body.
Phase 0 trials involve extremely small doses and very few participants. These studies are not designed to treat disease or improve health. Instead, researchers observe how the drug is absorbed, distributed, metabolized, and eliminated.
Because of the specialized testing required, Phase 0 trials are usually conducted at large academic hospitals rather than community research offices.
Phase 1 — Safety and Dosing Studies
Purpose: Identify a safe dose range and document early side effects.
A phase 1 trial (also called a phase I study or phase i trial) is the first time a medication or therapy is tested in humans. These trials often include:
- Small groups of participants
- Close physician supervision
- Gradual dose escalation
- Frequent lab testing and vital sign checks
Some phase 1 studies involve healthy volunteer paid clinical trials, while others enroll patients with serious diseases such as cancer, including advanced cases where standard treatments like chemotherapy are no longer effective.
These trials establish baseline safety before a drug can move forward in the drug trial phases.
Phase 2 — Does the Treatment Work?
Purpose: Evaluate whether the treatment appears effective for a specific condition.
A phase 2 clinical trial (also called a phase ii trial or trials phase 2) enrolls more patients than Phase 1 and focuses on treatment response. Researchers measure:
- Symptom improvement
- Lab or imaging changes
- Disease-specific outcomes
- Early efficacy signals
At Santos Research Center, we conduct Phase 2 research for a wide range of conditions, including diabetes, psoriasis, alopecia areata, infectious disease, and other skin conditions. These studies allow Tampa patients to access investigational medicine under close medical supervision.
This phase answers a key question in clinical research: does the treatment help?
Phase 3 — Large-Scale Comparison Studies
Purpose: Confirm effectiveness and compare the new treatment to standard care.
A phase iii clinical trial (also known as a phase 3 trial) involves hundreds or thousands of patients across multiple research sites. These studies typically include:
- Randomized assignments
- Blinded study designs
- Longer follow-up periods
- Comparisons to existing treatments or placebo
Phase 3 trials provide the data the FDA uses to evaluate approval for medications, vaccines, and medical devices. Many Phase 3 trials in Tampa, including those conducted at Santos Research Center, offer early access to promising treatments for a wide range of medical conditions.
Phase 4 — Long-Term Safety After FDA Approval
Purpose: Monitor real-world safety and effectiveness.
Once a medication is approved, Phase 4 trials study long-term outcomes in everyday health care settings. These trials may evaluate:
- Rare or delayed side effects
- Effectiveness across diverse patient populations
- Quality of life improvements
- Comparisons with newer treatments
Phase 4 research helps refine how approved medicine is prescribed in hospitals, clinics, and pharmacies. Visit our blog to learn more about how to join a clinical trial.
Other Ways Clinical Trials Are Categorized (Treatment, Prevention, and More)
In addition to clinical trial phases, studies are grouped by purpose:
- Treatment Trials: Test drugs, therapies, or procedures for disease management
- Prevention Trials: Evaluate vaccines or lifestyle approaches to reduce disease risk
- Diagnostic Trials: Study new tools to identify disease earlier
- Screening Trials: Determine better methods to detect disease
- Quality of Life Trials: Focus on symptom relief, daily function, and supportive care
These trial types can occur across different phases of clinical research depending on development stage. If you're new to clinical research, you can also review our guide on what a clinical trial is and how it works for a deeper look at how studies are structured.
How Study Designs Work
Clinical trials use structured study designs to reduce bias and ensure results are accurate and reliable.
Randomization
Participants are assigned to treatment groups by computer, not by the research team. This helps ensure groups are similar at the start of the study so outcomes reflect the treatment itself, not outside factors.
Blinding
Blinding helps prevent expectations from influencing results.
- Single-blind: Participants do not know which treatment they receive.
- Double-blind: Neither participants nor study staff know the treatment assignment.
- Open-label: Everyone knows which treatment is given, typically used when blinding is not practical or ethical.
Placebos
Placebos contain no active medication and are only used when it is safe and ethically appropriate. Participants are never denied necessary medical treatment, and placebo use is carefully reviewed and approved as part of the study design.
How Participant Safety Is Protected
Safety is central to every clinical trial at Santos Research Center.
- Institutional Review Board (IRB): Reviews protocols, consent forms, and risk-benefit balance
- FDA Oversight: Monitors safety data throughout drug development
- Data Safety Monitoring Boards: Independent experts reviewing ongoing results
- Informed Consent: Patients receive clear medical information before enrolling
- Ongoing Monitoring: Regular exams, labs, and physician oversight
- Institutional Review Board (IRB): Reviews protocols, consent forms, and risk-benefit balance
Participants may leave a study at any time without affecting their regular health care.
What Participation Looks Like in Each Phase
Every clinical trial phase has a different pace and experience. Here’s what most participants notice as they move from early-phase research to large-scale studies.
Phase 1: Short, Frequent Visits Focused on Safety
Phase 1 studies concentrate on safety monitoring.
Participants can expect:
- Frequent study visits
- More lab work and observation
- Extra monitoring after dosing
These trials are usually short and highly supervised.
Phase 2: Condition-Focused Testing and Symptom Tracking
Phase 2 evaluates how well the treatment works for a specific condition.
Participants typically experience:
- Visits similar to specialist appointments
- Symptom tracking, questionnaires, or imaging
- Randomized assignment to treatment, standard care, or placebo
These studies help determine whether the treatment shows meaningful benefit.
Phase 3: Structured Visits and Comparative Testing
Phase 3 involves more participants and longer study timelines.
Participants often see:
- Longer follow-up periods
- Larger study groups
- More specialized assessments (heart tests, imaging, detailed lab work)
These trials generate the evidence used for FDA approval decisions.
Phase 4: Routine Visits and Long-Term Monitoring
Phase 4 studies follow approved treatments in real-world use.
Participants usually have:
- Visits similar to routine office care
- Data collected from everyday medication use
- Long-term observation of benefits and rare side effects
These studies help refine how a treatment is used in everyday healthcare.
Participation Requirements by Phase (At a Glance)
| Phase | Visit Frequency | Typical Assessments | Study Duration |
|---|---|---|---|
| Phase 1 | Very frequent | Safety labs, vitals, observation | Short-term |
| Phase 2 | Regular | Symptom tracking, questionnaires, imaging | Weeks to months |
| Phase 3 | Scheduled | Imaging, heart tests, expanded lab panels | Months to years |
| Phase 4 | Routine | Long-term monitoring, everyday use data | Ongoing |
Compensation may be available depending on the study.
Common Myths About Clinical Trial Phases
Myth: Placebos are always used
Fact: Placebos are only used when it is ethical and medically appropriate. Many trials compare a new treatment to standard care rather than a placebo. When placebos are used, participants are never denied necessary medical treatment, and safety monitoring remains in place throughout the study.
Myth: Phase 1 trials are unregulated
Fact: Phase 1 trials follow strict FDA regulations and oversight. These studies require IRB approval, detailed safety protocols, and continuous monitoring. Dose escalation is carefully controlled, and participants are closely observed to identify side effects early.
Myth: You must complete every phase of a clinical trial
Fact: Most participants take part in only one phase of research. Clinical trial phases are designed for drug development, not patient progression. A participant may enroll in a Phase 2 or Phase 3 study without ever participating in earlier phases.
Myth: Larger trials are more dangerous
Fact: Phase 3 trials are often safer than earlier phases. By the time a study reaches Phase 3, the treatment has already undergone extensive safety testing. Larger trials benefit from established dosing, clearer risk profiles, and broader safety data collected across multiple study sites.
Why Participate in Clinical Trials at Santos Research Center?
Santos Research Center conducts Phase 2 and Phase 3 clinical trials in Tampa, offering:
- Access to investigational treatments
- Study-related medical care at no cost
- Experienced internal medicine and research physicians
- Compensation for time and travel in eligible trials
Our research office is located at 5927 Webb Rd, Tampa, FL 33615, serving patients throughout the Tampa Bay area.
Ready to Explore Current Research Opportunities?
You can browse open trials on our Studies page, call our office at (813) 249-9100, or contact us by email to learn more about enrolling.
By participating in a clinical trial, you support medical research that improves patient care locally and worldwide.
Frequently Asked Questions About Clinical Trial Phases
What is Phase 1 of a clinical trial?
Phase 1 is the first stage where a new treatment is tested in people. Researchers evaluate safety, side effects, and dosing using a small group of participants who are closely monitored.
Why do Phase 3 clinical trials take longer?
Phase 3 trials enroll more participants, require more visits, and compare new treatments to standard care or placebo. The larger data set takes longer to collect and analyze.
Are placebos safe in clinical trials?
Yes. Placebos are only used when it is ethically appropriate and safe. Participants are never denied essential care just to receive a placebo.
What’s the difference between Phase 2 and Phase 3?
Phase 2 tests whether a treatment works. Phase 3 confirms those results in larger groups and compares the treatment to standard options.
Can I leave a clinical trial after joining?
Yes. Participation is always voluntary, and you may withdraw at any time without affecting your regular medical care.
About Santos Research Center:
Santos Research Center is an FDA- and IRB-compliant clinical research facility in Tampa, Florida. Our team has conducted more than 400 clinical trials across neurology, psychiatry, infectious disease, urology, metabolic health, dermatology, and oncology, depending on study availability.
Last medically reviewed: January 2026