Clinical Trials

By Geetika Rao, MPH | Last Updated: March 15, 2021

Image by: Brett Dashevsky

What are clinical trials? 

Clinical trials are structured research studies performed on patients. They aim to evaluate a medical, surgical or behavioral intervention. Clinical trials are the primary method that researchers use to find out if a new therapy, like a new drug or medical device (i.e., a pacemaker or a Covid-19 vaccine) is safe and effective. Sometimes, a clinical trial is used to learn if a new treatment is more effective and/or has less harmful side effects than a treatment already present in the market.

There were more than 60,000 active clinical trials underway worldwide in 2018.


Key Takeaways

  • The FDA approves the start of a clinical trial after scientists conduct laboratory tests to evaluate a potential therapy’s safety and efficacy.

  • Clinical trials progress through four phases to test a treatment.

  • Participants in clinical trials are all voluntary and must meet eligibility to participate in a study.

  • Diversity of participants is crucial to ensure wider applicability of results.


Phases of Clinical Trials

Clinical trials of drugs are usually described based on their phase. The FDA typically requires Phase I, II and III trials to be conducted before determining if the treatment can be approved for use. If, after the first three phases, researchers find it to be safe and effective, the FDA approves it for clinical use and continues to monitor its effects in a Phase IV trial.

Phase I: "Is it Safe?"

Phase I trials focus on safety. A Phase I trial tests an experimental treatment on a small group of healthy people (20 to 100) to judge its safety and side effects and to find the appropriate drug dosage.

Approximately 70% of therapies move on to the next phase.

Phase II: "Does it Work?"

The emphasis in a Phase II trial is on effectiveness. This phase aims to obtain initial data on whether the drug works in people who have the specific illness or condition the drug claims to treat. These trials also continue to study safety, including short-term side effects. This phase can last several years and typically enrolls between 100-300 patients.

Approximately 33% of therapies move on to the next phase.

Phase III: "Is It the Same or Better?"

A Phase III trial continues to gather more information about safety and effectiveness. In this phase, different populations, dosages and drug interactions are investigated. The number of subjects usually ranges from 700 to about 3,000 people and can last for as long as 5 years. If the FDA agrees that the trial results are positive, it will approve the experimental drug or device for wider use.

Approximately 25% of therapies move on to the next phase.

Phase IV: "Can it Stick Around?"

A Phase IV trial for drugs or devices takes place after the FDA approves their use. A device or drug's effectiveness and safety are monitored in larger, more diverse populations. Sometimes, the side effects of a treatment may not become clear until more people have used it over a longer period of time, and so it is crucial to monitor utilization in the long term. 

Participation in Clinical Trials

Who Can Participate in a Clinical Trial?

Clinical studies have criteria outlining who can participate. These standards are listed in the research plan or protocol. Some phases seek participants who have the disease or condition that will be studied, other phases are looking for healthy participants, and some studies are limited to a predetermined group of people who are asked by researchers to enroll, or pre-selected by their doctors. 


The factors that allow someone to participate in a clinical study are called inclusion criteria, and the factors that disqualify someone from participating are called exclusion criteria. They are based on characteristics such as age, gender, the type and stage of a disease, previous treatment history and other medical conditions a participant may have.

How Are Participants Protected?

Informed consent is a process used by researchers to provide potential and enrolled participants with information about a clinical study. This information helps people decide whether they want to enroll in or continue to participate in the study. The informed consent process is intended to protect patients and should provide enough information for a person to understand the potential risks and benefits of the study, as well as alternatives to the study. Participants may withdraw from a study whenever they want, even if the study is not over. 

Diversity in Clinical Trials

It is important for clinical trials to have participants of different ages, sexes, races, and ethnicities and health conditions. If a clinical trial involves a group of participants who are too similar, the findings may not apply to or benefit a larger group of people. When clinical trials include diverse participants, the study results may have much wider applicability.


Outside the Huddle


Reviewed by Jared Dashevsky, M.Eng