How does a clinical trial work?

“I’ve heard clinical trials can provide alternative treatment options and help benefit advancements in medicine, but what exactly is clinical research and how does it work?”


These are questions we hear often as there can be many misconceptions about clinical trials and how they work. Part of CenterWatch’s mission is to help educate patients about clinical research to make a more informed decision about participation. So what are clinical trials and how do they work?

Clinical trials are conducted by medical doctors, or physician investigators, to collect data regarding the safety and efficacy of new drug development for the treatment of all types of medical conditions. There are several steps and stages of approval in the clinical trials process before a new drug can be considered safe and effective for sale to the general public, if ever.

Typically, clinical trials are conducted in four phases with each phase treated as a separate trial. Healthy volunteers or patients are selected based on meeting a defined set of criteria. The factors that allow volunteers to participate in a clinical trial are called “inclusion criteria” and the factors that disallow volunteers from participating are called “exclusion criteria.” These criteria can include age, gender, the type and stage of a disease, previous treatment history and other medical conditions.

After each phase is completed and prior to moving on to the next phase, data is submitted to the Food and Drug Administration (FDA) for approval.

  • Phase I assesses the safety and effects of a drug including: how it is absorbed, metabolized or excreted. This initial testing phase can take several months to complete and usually includes a small number of healthy volunteers ranging from 20 to 100 people and they may be paid for their participation. About 70% of drugs pass this phase.
  • Phase II tests the efficacy, or effectiveness, of a drug and how well it works on patients. This phase can last from several months to two years, and involves up to several hundred patients. Most phase II studies are randomized trials where one group of patients receives the experimental drug, while a second “control” group receives a standard treatment or placebo. Often these studies are “blinded” so that neither the patients nor the researchers know who has received the experimental drug. This allows investigators to provide the pharmaceutical company and the FDA with comparative information about the relative safety and effectiveness of the new drug. About 30% of drugs pass both Phase I and Phase II studies.
  • Phase III involves randomized and blind testing in several hundred to several thousand patients. This large-scale testing, which can last several years, provides the pharmaceutical company and the FDA with a more thorough understanding of the effectiveness of the drug, the benefits and the range of possible adverse reactions. Between 70% and 90% of drugs successfully complete this phase. Once completed, the pharmaceutical company can request FDA approval to sell the drug to the public.
  • Phase IV studies are conducted after a drug has been approved for consumer sale. At this stage, pharmaceutical companies compare the drug with other drugs already in the market, monitor a drug’s long-term effectiveness and impact on a patient’s quality of life and determine the cost-effectiveness of a drug therapy relative to other traditional and new therapies.

“Where can I learn more about participating in a clinical trial?”

Participating in a clinical trial is a personal and even a family decision. Research studies are very involved so it is important to learn as much as possible about the study you may participate in before you consent. Consider speaking with your doctor first or another medical professional. You can also visit our website at We provide a list of questions to ask your doctor along with information on informed consent, what to expect during the trial process and what happens post-trial.

“Where can I find clinical trials?”

Today, many clinical trials are posted online and can be found through a simple search. CenterWatch lists thousands of clinical trials – you can search by medical condition such as diabetes or COPD, by therapeutic area such as oncology or dermatology or by your location. Each trial is written in patient-friendly language so you don’t need to be a researcher to understand the details. You’ll also find basic qualifying or disqualifying criteria, location details and contact information. If you find a trial you are interested in, contact the research center noted to start the process and see if you qualify, or call us at 866-219-3440 and we’ll help you navigate the site and answer any questions you have.


Ryan Hall is a patient project representative for CenterWatch, and is committed to providing patients and their advocates with important information on clinical trials, FDA drug approvals and other essential health and educational resources.

If you’re interested in more information about clinical trial participation, visit our Patient Resource Center at:

If you have any additional questions about clinical trial participation, email, call toll-free at 866-219-3440 or visit us at