What are clinical trial phases?

Before becoming available to patients, every medication, therapy, and intervention must undergo a rigorous approval process: the clinical trial. Clinical trials are divided into phases that are aimed at testing the safety and efficacy of every potential new treatment before it is able to enter the market.

When researchers develop new drugs in a lab, they are initially in the discovery phase, which can take anywhere from three to six years. Before advancing the compound to human clinical trials, researchers will typically test the drug on animals to learn more about its impact. If the medication appears promising, researchers, pharmaceutical companies, or biotechs can begin enrolling volunteers in a phase I trial.

From discovery to approval, it typically takes approximately ten years for a new treatment to hit the market, with clinical trials making up six to seven years of that time. Each clinical trial is split into distinct phases, which we will outline below. 

Phase I clinical trials

Phase I clinical trials test whether a medication is safe for human use and can last anywhere from several months to a year. These trials are typically small, enrolling around 20 to 100 volunteers. Some phase I trials only enroll healthy volunteers who are often compensated for participating. Other trials may accept participants with the condition the drug is aiming to target. 

At the beginning of the phase, doctors will provide a very small dose of the drug and measure how it is absorbed, metabolized, and excreted by the body, and will also monitor for side effects. In phase I, the goal is to prove safety — while the drug may help symptoms, a phase I trial is not intended to showcase how effective the therapy is. If researchers find the drug to be safe, it will then progress to phase II. Around 70% of experimental drugs move past this stage. 

Phase II clinical trials

Phase II clinical trials test both the safety and effectiveness of a drug or medical device. Trials in this stage can last from several months to a few years, and recruit up to several hundred patients with the condition to take part.

Phase II clinical trials are typically randomized, so one group of participants will receive the study drug and the other group will receive a placebo or the current standard of treatment. Phase II studies can also be blinded, which means that neither the researchers nor the participants know who received the study drug. To move to phase III, the therapy has to be deemed both safe and effective in the participants.

Phase III clinical trials

Phase III clinical trials usually enroll a high number of people and are both randomized and blinded. This testing phase can last several years as the FDA gathers thorough data about the drug's effectiveness and potential side effects in a larger pool of participants.

After a phase III trial, a pharmaceutical company can request FDA approval to start marketing the drug. A pharmaceutical company may submit a New Drug Application (NDA) or a biologics license application (BLA) to the FDA. The FDA then reviews results from all stages of the trial to determine whether it will approve the drug and allow the pharmaceutical company to begin marketing it to the public. The FDA may also require that additional research be conducted, or call a hearing to determine whether the drug’s benefits outweigh its risks.

In the event of a public health emergency such as the COVID-19 pandemic, the FDA is able to issue an Emergency Use Authorization (EUA). This is a mechanism to facilitate the availability and use of medical countermeasures, including vaccines. While the process for COVID-19 vaccines was and continues to be greatly accelerated, the clinical trials being conducted adhere to the same rigorous standards set forth by the FDA for other potential treatments and devices. 

Phase IV clinical trials

Also known as “Post-Approval Research and Monitoring,” Phase IV takes place after a drug or device has been approved. In this phase, pharmaceutical companies monitor a drug's long-term impact and any side effects that may result from continued use of a medication.

Is a clinical trial right for you?

If you’re considering participating in a research study, it is a good idea to ask the study team about the clinical trial timeline. Some trials may last for several years, but only require visits every few months, for example. Other trials may require more frequent visits, or for participants to make entries in an e-diary or paper diary in-between visits. You'll want to weigh the potential benefits of participation against the time it may require of you. Some clinical trials reimburse you for participating in a trial, and others pay for travel.

The clinical trial process is long, but this is by design. The length of a clinical trial is designed so that drugs have been thoroughly evaluated by the time they reach the market — but this is only possible when volunteers take part. Up to 80% of clinical trials are delayed due to a lack of participants, so if you’d like to take part in research, click the button below.