A clinical study is a research project involving human volunteers that follows a written plan to answer a specific question about health or medical care. Researchers use clinical studies to learn how conditions develop and how people respond to treatment. Every prescription medication on the market today, including the antidepressants and anti-anxiety medications many people rely on, reached patients only after it was tested in clinical studies.
If you have been invited to join a study or are simply curious about how medical research works, this guide explains the main types of clinical studies, the phases a new treatment moves through, and what participation looks like from the first screening visit to the final follow-up.
Key takeaways
A clinical study is research that involves people and is designed to answer specific health questions. The two main types are clinical trials, which test an intervention such as a new medication, and observational studies, which track health outcomes without assigning treatment. New drugs pass through several trial phases before FDA review.
What Is a Clinical Study?
“Clinical study” is the umbrella term for any medical research that involves people. Lab experiments and animal research can show whether an idea is promising, but only studies in people can show how a treatment performs in the patients it is meant to help.
According to ClinicalTrials.gov, the federal registry run by the National Library of Medicine, clinical studies fall into two broad categories: interventional studies (usually called clinical trials) and observational studies. The difference comes down to one question. Do the researchers decide what treatment a participant receives, or do they simply watch and record what happens?
Clinical Trials vs. Observational Studies
In a clinical trial, researchers assign participants to receive a specific intervention according to a detailed research plan called a protocol. The intervention might be a medication, a medical device, a procedure, a form of therapy, or a change in behavior such as a new exercise routine.
In an observational study, participants receive whatever care they and their own doctors choose. Researchers collect information about their health over time, but they do not assign any treatment.
| Feature | Clinical trial | Observational study |
| Who decides the treatment | The research protocol | The participant and their doctor |
| Main question | Does this intervention work, and is it safe? | What factors are linked to a health outcome? |
| Comparison group | Often uses a placebo or standard treatment | Compares people with different exposures or histories |
| Can show cause and effect | Yes, when well designed and randomized | Shows associations; other factors may explain the link |
| Example | Testing a new medication for depression against a placebo | Following a group of adults for years to see who develops anxiety |
Types of Clinical Trials
Clinical trials are grouped by their primary purpose. ClinicalTrials.gov uses these categories when researchers register a study:
- Treatment trials test new medications, therapies, or devices, or new ways of combining existing treatments.
- Prevention trials look for ways to keep a condition from starting or returning. A study of whether ongoing therapy lowers the chance of a depression relapse is one example.
- Diagnostic trials evaluate new tests or procedures for identifying a condition more accurately.
- Screening trials study ways to detect a condition early, sometimes before symptoms appear.
- Supportive care trials explore ways to improve comfort and quality of life, such as managing treatment side effects.
- Basic science trials examine how an intervention works inside the body, for example how a medication changes activity in certain areas of the brain. Their findings often shape the design of later treatment trials.
In mental health research, interventions can include medications, structured talk therapy programs, and brain stimulation techniques such as transcranial magnetic stimulation (TMS), a treatment that uses magnetic pulses to stimulate nerve cells in the brain.
Types of Observational Studies
Observational studies help researchers spot patterns in large groups of people. A review of observational research methods published in the Emergency Medicine Journal in 2003 describes the designs below, each with its own strengths and limits.
Cohort Studies
A cohort study follows a group of people over time and compares those who have a particular exposure or trait with those who do not. Researchers might follow 5,000 adults for a decade to see whether poor sleep is linked to a higher rate of depression later on.
Because cohort studies track events in the order they happen, they can show which came first and measure how often new cases develop. They can also be expensive and slow, and participants may drop out over the years. Like all observational research, cohort studies show associations. An unmeasured factor, such as chronic stress, could explain both the exposure and the outcome.
Case-Control Studies
A case-control study starts with people who already have a condition (the cases) and compares them with similar people who do not (the controls). Researchers then look back in time to see which past exposures or experiences differ between the two groups.
These studies are relatively fast and affordable, and they work well for rare conditions. Their main weaknesses are recall bias, since people with a condition may remember past events differently from people without it, and the difficulty of choosing a control group that is truly comparable.
Nested Case-Control Studies
A nested case-control study takes place inside an existing cohort. As members of the cohort develop the condition being studied, they become cases, and researchers select matched controls from the same cohort who have not developed it.
Because information about everyone was collected before anyone was diagnosed, this design reduces recall bias. It also lets researchers study a large cohort efficiently by analyzing detailed data, such as stored blood samples, for a smaller group of people.
Cross-Sectional Studies
A cross-sectional study captures a snapshot of a population at a single point in time. A survey measuring how many college students report anxiety symptoms this semester is a cross-sectional study. These studies are useful for measuring how common a condition is, but they cannot show whether an exposure came before or after the condition developed.
The Phases of a Clinical Trial
New medications are tested in a series of phases, with each phase building on the results of the one before it. The U.S. Food and Drug Administration outlines the typical size, length, and purpose of each phase.
| Phase | Typical number of participants | Typical length | Main purpose |
| Phase 1 | 20 to 100 healthy volunteers or people with the condition | Several months | Safety and dosage |
| Phase 2 | Up to several hundred people with the condition | Several months to 2 years | Effectiveness and side effects |
| Phase 3 | 300 to 3,000 people with the condition | 1 to 4 years | Effectiveness and monitoring of adverse reactions |
| Phase 4 | Several thousand people with the condition | Ongoing after approval | Long-term safety and effectiveness in the wider population |
Some drug development programs also begin with a small exploratory study, sometimes called Phase 0, that gives a few participants very low doses to learn how a drug behaves in the body.
Many treatments do not make it through every phase. The FDA estimates that about 33% of drugs move from Phase 2 to Phase 3, often because a treatment did not work well enough or caused too many side effects. That attrition is part of how the system protects patients, since only treatments with solid evidence reach FDA review.
Phase 4 studies happen after a treatment is approved and on the market. They track how it performs across a larger and more varied group of people over a longer period, which can reveal rare side effects that smaller trials missed.
How a Clinical Study Works, Step by Step
Every study follows its own protocol, but most clinical trials move through a similar sequence.
- The protocol is written and reviewed. Researchers write a protocol that spells out who can join, which tests and treatments participants receive, the visit schedule, and how long the study lasts. An independent ethics committee must approve the protocol before anyone enrolls.
- Volunteers are screened for eligibility. Each study has inclusion and exclusion criteria, which can cover age, diagnosis, symptom severity, current medications, and other health conditions. A screening visit may include an interview, questionnaires, a physical exam, and lab work.
- You give informed consent. The study team explains the purpose of the study, what will happen at each visit, the possible risks and benefits, and other treatment options available to you. You get time to ask questions and talk it over with family or your own doctor before signing. The National Institutes of Health describes informed consent as an ongoing process that continues for as long as you are in the study.
- Participants are assigned to groups. In a randomized trial, a computer assigns each person to a group by chance, which keeps the groups comparable. One group may receive the study treatment while another receives a placebo (an inactive substance that looks like the real treatment) or a standard treatment. In a double-blind study, neither participants nor the research staff know who is in which group until the study ends, which prevents expectations from influencing the results.
- Study visits and monitoring take place. Participants attend scheduled visits where the team checks symptoms, reviews side effects, and may take vital signs or blood samples. Visits give researchers consistent data and give participants regular contact with the study team.
- Follow-up and results. After the treatment period ends, some studies include follow-up visits to check how participants are doing. Researchers then analyze the data, and many trials are required to post a summary of results on ClinicalTrials.gov.
Who Oversees Participant Safety
Several layers of oversight protect people who take part in clinical research.
Institutional review boards (IRBs) are independent committees of physicians, scientists, and community members. An IRB reviews the study plan before it begins and continues to review it while the study runs, with the goal of making sure risks are minimized and reasonable in relation to the knowledge the study could produce.
Data and safety monitoring boards are used in many larger trials. These independent experts review results as they come in and can recommend changing or stopping a study early if safety concerns appear.
The FDA oversees clinical trials of medications and medical devices in the United States, including reviewing the safety data that supports each phase.
You also keep the right to leave a study at any time, for any reason, without it affecting your access to regular medical care.
These safeguards reduce risk, although they cannot remove it entirely. Every study carries some possible risks, and the consent process is where the study team explains them in detail. For a closer look at weighing those risks against the potential benefits, read our guide on why participate in clinical trials.
Clinical Studies in Mental Health Research
Mental health conditions such as depression, anxiety, bipolar disorder, PTSD, and schizophrenia are active areas of clinical research. The National Institute of Mental Health funds and conducts studies across many of these conditions and maintains resources for people considering whether to volunteer.
Psychiatric studies look a bit different from studies in other areas of medicine. There is no blood test that confirms depression or measures how much it has improved, so researchers rely on validated rating scales. Trained clinicians ask a standard set of questions and score the answers the same way at every visit. One of the most widely used, the Hamilton Depression Rating Scale, was first published in the Journal of Neurology, Neurosurgery, and Psychiatry in 1960, and versions of it are still used in depression trials today. Participants often complete self-report questionnaires as well, so the data reflects both the clinician’s view and their own experience.
Many mental health studies also include regular safety check-ins that ask about changes in mood, sleep, and thoughts of self-harm. These questions are a standard part of monitoring and give the study team a chance to respond quickly if symptoms worsen.
If you currently take medication for a mental health condition, the protocol will explain how your existing treatment fits into the study. Bring a complete list of your medications to the screening visit, and do not stop or change any medication without talking to your prescriber.
How to Find a Clinical Study
ClinicalTrials.gov lists studies conducted in the United States and around the world. You can search by condition and location, then filter for studies that are currently recruiting. Each listing includes the study’s purpose, phase, eligibility criteria, and contact information.
Your own doctor is another good starting point, since they know your health history and can help you judge whether a study is a reasonable fit. Research centers also post their open studies directly. You can browse the current studies at the Lehigh Center for Clinical Research to see what is enrolling now.
Study details, including eligibility and enrollment status, can change as a study progresses. Always confirm the latest information with the study team.
Frequently Asked Questions About Clinical Studies
Is a clinical study the same as a clinical trial?
A clinical trial is one type of clinical study. “Clinical study” covers all research involving people, including observational studies where no treatment is assigned. A clinical trial specifically tests an intervention, such as a medication or therapy, according to a research protocol.
What is a placebo, and will I receive one?
A placebo is an inactive substance made to look like the treatment being tested. Some trials use a placebo group so researchers can tell whether improvements come from the treatment itself. Not every trial uses one, and the consent form will tell you whether yours does and what your chances are of being assigned to each group.
Can I leave a clinical study after I join?
Yes. You can withdraw from a study at any time, for any reason, and it will not affect your regular medical care. Let the study team know before you stop, especially if you are taking a study medication, so they can help you stop safely.
Do I need a diagnosis to join a clinical study?
It depends on the study. Many treatment trials require a confirmed diagnosis and a certain level of symptoms, which the team checks during screening. Some Phase 1 trials and observational studies enroll healthy volunteers.
How long does a clinical study last?
Study length ranges from a few weeks to several years, depending on the phase and design. The protocol lists the full schedule of visits, and the study team will walk you through it before you agree to take part.
Exploring Current Studies at the Lehigh Center for Clinical Research
Clinical studies are how medicine learns what works, for whom, and at what dose. Each one depends on volunteers who choose to take part, and each one is built around rules that put participant safety first.
If you would like to learn whether a study might be a fit for you, view our open clinical studies or contact our research team with your questions. We are glad to explain what a specific study involves so you can make an informed decision alongside your doctor.
Medical disclaimer: This article is for general educational purposes and is not a substitute for professional medical advice, diagnosis, or treatment. Always talk with a qualified healthcare provider about your individual health questions and before making any changes to your care. If you are in crisis or thinking about harming yourself, call or text 988 to reach the 988 Suicide & Crisis Lifeline.
Sources
- ClinicalTrials.gov, National Library of Medicine. Learn About Clinical Studies.
- U.S. Food and Drug Administration. Step 3: Clinical Research.
- National Institutes of Health. NIH Clinical Research Trials and You: The Basics.
- National Institute of Mental Health. Clinical Trials and You.
- Mann CJ. Observational research methods. Research design II: cohort, cross sectional, and case-control studies. Emergency Medicine Journal. 2003;20(1):54-60. doi:10.1136/emj.20.1.54.
- Hamilton M. A rating scale for depression. Journal of Neurology, Neurosurgery, and Psychiatry. 1960;23(1):56-62. doi:10.1136/jnnp.23.1.56.
