4.How Do Clinical Trials Work?


If you take part in a clinical trial, you may get tests or treatments in a hospital, clinic, and/or doctor’s office.

In some ways, taking part in a clinical trial is different from having regular care from your own doctor. For example, you may have more tests and medical exams than you would otherwise.

The purpose of clinical trials is research, so the studies follow strict scientific standards. These standards protect patients and help produce reliable study results.

Clinical Trial Protocol

Each clinical trial has a master plan called a protocol (PRO-to-kol). This plan explains how the trial will work. The trial is led by a principal investigator (PI), who often is a doctor. The PI prepares the protocol for the clinical trial.

The protocol outlines what will be done during the clinical trial and why. Each medical center that does the study uses the same protocol.

Key information in a protocol includes:

  • How many patients will take part in the clinical trial
  • Who is eligible to take part in the clinical trial
  • What tests patients will get and how often they will get them
  • What type of data will be collected during the clinical trial
  • Detailed information about the treatment plan

Eligibility Criteria

A clinical trial’s protocol describes what types of patients are able to take part in the research—that is, who is eligible. Each trial must include only people who fit the patient traits for that study (the eligibility criteria).

Eligibility criteria differ from trial to trial. They include factors such as a patient’s age and gender, the type and stage of disease, and whether the patient has had certain treatments or has other health problems.

Eligibility criteria ensure that new approaches are tested on similar groups of people. This makes it clear to whom a clinical trial’s results apply. These criteria also are a safety measure. They ensure a trial excludes any people for whom the protocol has known risks that outweigh any possible benefits.

Clinical Trial Phases

Clinical trials of new medicines or medical devices are done in phases. These phases have different purposes and help researchers answer different questions.

For example, phase I clinical trials test new treatments in small groups of people for safety and side effects. Phase II clinical trials look at how well treatments work and further review these treatments for safety.

Phase III clinical trials use larger groups of people to confirm how well treatments work, further examine side effects, and compare new treatments with other available treatments.

Steps To Avoid Bias

The researchers doing clinical trials take steps to avoid bias. “Bias” means that human choices or other factors not related to the protocol affect the trial’s results.

Comparison Groups

In most clinical trials, researchers use comparison groups. This means that the patients taking part in a trial are assigned to one of two or more similar groups. Each group will receive different medical strategies.

For example, one group may get the current standard treatment for a condition, while another group gets a new treatment. Researchers can then compare the results to see whether one group has better outcomes than the other.

Using comparison groups also ensures that no one in a study is left without treatment for the sake of research.

Sometimes, when no treatment exists for a condition, people in one group may receive a placebo (plah-SE-bo). This is an inactive product that looks like the test product. You’ll be told if a placebo will be used in a study before you agree to take part.


Most clinical trials that have comparison groups use randomization. This involves assigning patients to different comparison groups by chance, rather than choice.

This method helps ensure that any differences observed during a trial are due to the different strategies being used, not to preexisting differences between the patients. Usually, a computer program makes the group assignments.


The term “masking” refers to not telling the clinical trial participants which treatment they’re getting. Masking, or “blinding,” helps avoid bias. For this reason, researchers also may not be told which treatments the patients are getting. However, trial records can quickly show this information if safety issues arise.

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