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Evaluating the Effects of a Synbiotic Versus Probiotic or Placebo on Gut Microbiota and Cardiometabolic Health: A Randomized Double-Blind Placebo Controlled Trial
What is a healthy volunteer?
A healthy volunteer is a person who does not have the disease or condition a study is looking at. That does not always mean perfect health. It means you do not have the condition the study is about, and you meet the rules the study sets, called eligibility criteria.
Researchers need healthy people for two reasons. Before a new drug goes to patients, they need to know what dose is safe and how long the drug stays in the body, and that is easier to see in people who are not sick and not taking other medicines. Cancer drugs are the exception: their first studies are done in people who have cancer. And many studies of a disease need healthy people to compare against, so researchers can see what is different. Before a new drug study can start, the FDA reviews the plan, and it has 30 days to do so. An independent ethics board checks the study too.
Types of healthy volunteer studies
Phase 1 drug studies. The first tests of a new medicine in people, to check safety and find the right dose. Many include one or more nights at a research unit.
Bioequivalence studies. These check that a generic medicine gets into the blood the same way as the brand name version. They usually need a few dozen healthy volunteers.
Vaccine studies. Vaccines are made for healthy people, so early vaccine trials are done in healthy adults. See our vaccine clinical trials page.
Sleep studies. Research on what sleep loss, shift work, or the body clock do to health. Some take place in a sleep lab for a few nights or even weeks. Our guide explains how paid sleep studies work.
Nutrition and exercise studies. Tests of foods, diets, supplements, and workouts. You often stay at home and come in for some visits.
Imaging and brain studies. MRI or PET scans that show how a healthy brain or body works. Many give no medicine at all.
Comparison group studies. Studies of a disease that also need healthy people for comparison. You get the same tests as the patients, but you do not have the disease.
What "accepts healthy volunteers" really means
On ClinicalTrials.gov, a study can mark that it accepts healthy volunteers. It means people who do not have the condition being studied are allowed to join. It does not mean the study is mostly for healthy people. Very often the healthy people are the comparison group in a study about a disease, and that is why a Parkinson's or a depression study can show up in a healthy volunteer search.
It is surprising, but in September 2026 there were about 5,500 recruiting studies in the US and Canada that accepted healthy volunteers, and fewer than 1 in 10 of them were Phase 1 drug studies. So read the title and the study details. Words like "healthy adults," "healthy participants," or "Phase 1" usually mean the study is built around healthy people.
Who can join a healthy volunteer study?
Every study has its own rules, but many healthy volunteer studies look for the same things. Most take adults 18 and older, often up to about 55 or 65, and some studies are only for people over 65. Many ask for a body mass index (BMI) in a middle range, often about 18 to 32. You may need to be a nonsmoker, to not take regular prescription medicines, and to have normal results on blood tests and a basic health check. Many studies test for drugs and alcohol, and some do not accept people who are pregnant or breastfeeding. If you were in another study recently, you may need to wait before you can join a new one. Eligibility always varies by study.
Considering volunteering in a clinical trial? Explore our comprehensive guide to learn more about why people choose to volunteer and the impact you can make in advancing medical research. Discover the process of volunteering, what to consider before applying and the potential benefits and risks involved.
What to expect in a healthy volunteer study
Screening first. A visit with health questions, a physical exam, blood and urine tests, and often a heart test (ECG). It checks whether taking part is a good fit for your health and whether you meet the rules.
Informed consent before anything else. The study team explains the purpose, every procedure, the known risks, and what each group in the study gets, and you can ask as many questions as you want. You sign only if you agree, and you can leave the study at any time. Our guide to informed consent explains what to ask.
Sometimes overnight stays. Some Phase 1 studies ask you to stay at a research unit for one night or more, sometimes a week or longer. You follow a set schedule for meals, sleep, and blood draws, and you usually stay inside the unit until the stay ends.
Follow up visits. After the main part, there are usually return visits or phone calls to check on you, sometimes for several weeks.
Can you join more than one study?
Usually not at the same time. Most studies do not let you be in two drug studies at once, because two medicines could affect each other and mix up the results. Many also ask you to wait after your last study before you start a new one. This gap is called a washout period. In Phase 1 studies it is often 30 days, and sometimes 90 days or longer. Tell the study team about any recent study at screening, it is there to keep you safe. Read more in our guide to washout periods.
Are clinical trials safe for healthy volunteers?
Every study carries some risk. Before a new drug study starts, the FDA reviews the plan and an ethics board checks it, and volunteers are watched closely during the study. The study team must explain the known risks during informed consent, and you can leave at any time.
Worried about the risks of participating in a clinical trial? Delve into our comprehensive guide to gain insights into the potential outcomes of participating. Learn about potential risks, benefits, and safety considerations to make an informed decision about your participation in a clinical trial.
Do healthy volunteers get paid?
Some studies offer compensation for time and travel, and study related tests and visits are typically provided at no cost to participants. Compensation varies by trial and is always described during the informed consent process before you agree to anything. Our guide to how clinical trial payment works has more detail.
How to enroll in a healthy volunteer study
Find your study. Use the filters and the toggle "Show studies for healthy volunteers only." Look for "healthy," "healthy adults," or "Phase 1" in the title to find studies built around healthy people.
Choose a location. Enter your city or state. Studies with overnight stays run at research units, so check how far the site is and whether you can be there on the study dates.
Read the study details. Click "Learn More" for the age range, BMI range, how many visits and nights, and any rules about smoking, medicines, or recent studies.
Complete the health profile. Click "Get started" to begin the 5-step application. Have three things ready: a list of all medicines and supplements you take, the dates of any clinical study you joined in the past year, and the days or nights you could be available.
Submit the application. A clinical trial coordinator reviews it and contacts you. Nothing is decided until you have gone through informed consent, and participation is voluntary at every step.