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Clinical Study Types - for Librarians

Phase I-IV: what do these phases mean?

Clinical trials are conducted in a series of steps called “phases.” Each phase has a different purpose and helps researchers answer different questions. The National Institutes of Health describes the phases as shown below.

  • Phase I trials: Researchers test a drug or treatment in a small group of people (20–80) for the first time. The purpose is to study the drug or treatment to learn about safety and identify side effects.
  • Phase II trials: The new drug or treatment is given to a larger group of people (100–300) to determine its effectiveness and to further study its safety.
  • Phase III trials: The new drug or treatment is given to large groups of people (1,000–3,000) to confirm its effectiveness, monitor side effects, compare it with standard or similar treatments, and collect information that will allow the new drug or treatment to be used safely.
  • Phase IV trials: After a drug is approved by the FDA and made available to the public, researchers track its safety in the general population, seeking more information about a drug or treatment’s benefits, and optimal use.

Note:  sometimes you will see a "Phase 0" or "Phase V" study.  These are variations on the above;  Phase I-IV is the standard.

                                                                                                                               Image from USCD

How to Find Phase I-IV Trials in PubMed using MeSH

In PubMed, Phase I-IV trials can be searched separately.   When you search in MeSH, first you will see that PubMed splits these into two areas:  As Topic and Publication Type.  Which to use?  If you're doing a systematic review, use [publication type].   There is no category for "Phase I-IV trials."  You will have to search separately on each phase that you are interested in, and combine them using OR.

image from MeSH database

                                  Images MeSH database, Phase I As TopicMeSH database, Phase I [Publication Type]

Basically, "Publication Type"  means it's an article about the results of a trial.  "As Topic" means it's about the general topic of trials: maybe how to recruit patients, or how to conduct a specific aspect of a trial.

Examples of articles that were tagged with each MeSH term:

  • Publication Type: (these are probably the types of articles you want)
    • Phase I clinical trial of percutaneous cryoablation for osteoid osteoma
    • Phase I trial of everolimus, gemcitabine and cisplatin in patients with solid tumors.
  • As Topic: (you probably don't want these articles returned in your search results)
    • Evaluation of Deviation From Planned Cohort Size and Operating Characteristics of Phase 1 Trials.
    • Patient-Centered, Physician-Investigator Friendly Pragmatic Phase Trial Designs-The 4P Model.



National Institutes of Health.NIH Clinical Trials Research and You: The Basics.

University of California San Diego Medical School, Huntington Disease Clinical Research Center.Cliinical Studies Explained.

National Center for Biomedical Information, MeSH Database.Clinical Trials, Phase I-IV.

National Center for Biomedical Information, MeSH Database.Phase I Trials, Publication Type.

National Center for Biomedical Information, MeSH Database.Phase I Trials, As Topic.

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