Depending on the clinical trial or observational study, the participants may be people who have been diagnosed with a particular disease, are survivors of a disease, are at heightened risk of a disease or are simply healthy volunteers. Some people participate in a clinical trial because there are no standard treatments available to them or because other treatments they have tried haven’t worked. Others participate for altruistic reasons, such as to advance medical knowledge. People who volunteer for lifestyle and prevention studies may be genuinely interested in learning ways to improve their health and reduce their risk of disease.

By participating in a clinical trial, volunteers have the chance to try a new intervention that is generally not available outside of the study. If the treatment is more effective than standard care, participants may be among the first to benefit from it. In some clinical trials and observational studies, participants gain information about their health status and health risks that they wouldn’t otherwise have access to. Some lifestyle studies include consultation with a nutritionist or exercise specialist or access to innovative strategies for smoking cessation or weight loss that may prove beneficial.

Clinical trials come with certain risks. New interventions being studied in a clinical trial may or may not be as effective or safe as standard treatments and care. Indeed, in early clinical studies, the safety, side effects and effectiveness of the intervention are still being ascertained. In randomized studies, patients cannot choose which group they are assigned to; in blinded studies, patients do not know which intervention they are receiving (or, in some cases, whether they are receiving a placebo).
Many measures are taken to protect the safety of clinical trial participants, including careful study design, oversight by institutional review boards and periodic monitoring of study data by independent experts. Participants also receive close medical attention and observation. Researchers must inform participants of any new risks, benefits or side effects they discover.

Participation in a clinical trial may involve diagnostic tests, treatments, procedures, doctor visits or hospital stays that are not part of standard care. Preventive studies may involve making changes to diet, exercise or other aspects of lifestyle. Some observational studies involve minimal effort, such as agreeing to share certain test results, maintaining a diet or exercise log, or filling out questionnaires.

Placebos, which do not contain any known active ingredient, may be used in clinical trials, but not if it would mean putting participants at risk by denying them effective therapy. This is particularly true for cancer and other serious illnesses, where placebos are rarely used. You can find out in advance if a study you are considering might involve a placebo.

Some health insurance providers will pay some or all of the costs associated with participating in a clinical trial. Most often, the study itself covers the cost of the intervention while the insurance provider pays for routine care. You can ask the study staff and your insurance provider in advance to determine which costs are likely to be covered.

The descriptions of clinical trials and observational studies clearly spell out who is potentially eligible. Criteria may include age, gender, ethnicity, type and stage of disease, or other characteristics. Many studies also spell out medical conditions that may exclude a person from eligibility.

Each study will specify how to enroll — such as by contacting the researchers via email or filling out an online form. The researchers will review each person’s information to determine eligibility and may ask prospective participants to come in for a screening visit or take part in a phone interview.

Some studies pay volunteers for their participation — particularly lifestyle studies and studies that involve healthy volunteers. This will be spelled out clearly in the study description. Depending on the number of clinic visits and the duration of the study, volunteers may be paid up to a few hundred dollars upon completion of the activities.