What is a Phase II Clinical Trial?

Clinical research must go through different phases for market approval. After the pre-clinical research has been completed in the lab, the first stage of clinical testing in humans is called Phase I, which is the initial testing of a new drug, treatment, or medical device to determine if it is safe to use in humans. Not all clinical research will move on to the other phases, but if the Phase I results are positive, a trial will typically move from Phase I into Phase II. Phase II clinical trials are an essential step in the drug development process. This article will explain how Phase II clinical trials work and what happens during a Phase II trial and after it’s completed.

What is a Phase II clinical trial?

Phase II clinical trials are the second stage in the development of a new drug, treatment, or medical device using human study participants. Phase II trials involve testing in a larger group (usually 100-300 people) to determine if the drug, treatment, or medical device is safe and effective before moving on to Phase III trials.

Phase II studies are also called Phase IIa studies, Phase 2a studies, and sometimes Phase 2b trials if they are conducted after one or more Phase 1b trials are completed. Phase II clinical trials can take place at different sites around the world and generally take about 2-3 years to complete, but can be completed in several months.

What is the purpose of a Phase II clinical trial?

Phase II trials are designed to test whether a drug, treatment, or medical device’s effects can be replicated in a larger group of people at multiple sites (called multi-institutional). They are also used to determine the optimal dosage for future trials and whether the drug has any potential adverse interactions with other medications. Phase II studies will also explore if any adverse side effects were not seen during the previous Phase I studies.

Phase II main goals:

  • To test the safety and efficacy of a drug, treatment, or device in a larger group of patients.
  • Monitor the effect in human study participants.
  • Learn more about potential side effects.
  • Discover optimal dosage.

What is involved in a Phase II clinical trial?

Clinical research must first pass through a Phase I clinical trial before a Phase II trial can start. Next, the sponsor would likely select a Contract Research Organization (CRO) to manage the study and sites that will conduct the clinical trial. Study participants will then be recruited through various methods and will be guided through every step of the study by the research staff at a site. Site staff will ensure that study participants understand their rights and responsibilities and what they can expect throughout the clinical trial process.

A large majority of Phase II studies are randomized, which means that participants are assigned to a treatment group or “arm” randomly (by chance, not by choice) to receive either the experimental drug, a standard of care treatment, or a placebo (an inactive substance). Those who receive the standard of care treatment or placebo are called a control group. Participants may also receive different doses of the treatment for researchers to gain a better understanding of the optimal dosing for the drug being studied.

In order for companies to complete a Phase II trial successfully, there must be seamless coordination between team members:

  • Clinical trial design specialists who create protocols for how each study will be conducted
  • Data analysts who manage all of the data collected during the trial
  • Regulatory affairs personnel who ensure compliance with industry guidelines and government regulations, such as Good Clinical Practices (GCPs)

The CRO, site staff, and sponsor must stay in communication and work together to avoid potential trial disruptions and to ensure the safety of the study participants enrolled in the clinical trial.

What happens after a Phase II clinical trial?

After Phase II, if the trial results were positive, the next trial (Phase III) would most likely be designed and implemented. Phase III is the last phase before marketing a drug as safe and effective for use by the general population.

In Phase III, researchers will study how well the drug works compared to other treatments available. They’ll also look at the proper dosing for the drug or treatment and if there are any side effects or interactions with other medications that should be noted.

When all testing has been done and reviewed by experts around the world, an independent panel of experts from different countries called “Regulatory Authorities” will decide whether or not to market the new medicine as safe and effective. In the United States, the regulatory authority is the U.S. Food and Drug Administration (U.S. FDA).

What is the role of a Contract Research Organization (CRO)?

A Contract Research Organization (CRO) is a crucial partner for biotech sponsors. CROs can be involved in all phases of clinical trials, including but not limited to:

  • Conducting and managing studies
  • Providing statistical analysis
  • Data management services
  • Writing protocol documents
  • Regulatory affairs assistance
  • Auditing sites for compliance with Good Clinical Practice (GCP) guidelines and other federal regulations
  • Training site staff on study procedures
  • Assisting with adverse event reporting systems
  • Monitoring patient recruitment at individual sites.

What CRO services do Phase II clinical trials need?

In Phase II trials, it’s important to note that the services provided by the CRO will be similar to those provided in the Phase I trials. However, in a Phase II trial, you usually have a larger sample size of study participants than in Phase I (generally at least 30 participants). This means that your clinical trial will cost more and may take longer to complete.

Patient recruitment efforts will need to be increased, and it will be vital for your CRO to have an excellent patient recruitment playbook. You also need randomization for your study design; otherwise, there could be too much variability in your results from participant to participant due to uncontrollable factors like age and gender. CROs can help with the management of this.


The goal of a Phase II clinical trial is to determine whether a drug has the potential to be effective and safe before moving on to a Phase III trial. Researchers need to obtain high-quality data from the Phase II trial to analyze the information to make these decisions for Phase III. To ensure a smooth trial with accurate data, finding the right CRO for your Phase II clinical trial will be important.

Vial is a full-service, tech-enabled CRO backed by a team of experts in clinical operations. Vial’s powerful technology platform allows for streamlined processes, faster recruitment, and organized data management all in one seamless system. Vial operates globally across therapeutic areas and Phases I-V.

Are you planning on running a Phase II clinical trial? We’d love to hear about it. Let’s connect!

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