Phases of Clinical Trials

Education Session

Our mission is to continue to provide our Sickle Cell community with updated information about Sickle Cell disease and other topics that promote healthy living in body and mind. Special Thank you to Terry Anderson , PhD, who came to our March education session and talked with us about Clinical Trials.


Clinical trials are an essential part of the drug development process and are the mechanism by which molecules are created in to medications. There are several phases of the clinical trial process, which are highly regulated by regulatory agencies and are required to bring a medication to market.

Preclinical-Testing of drugs to evaluate toxicity, pharmacokinetics (how the drug is absorbed, metabolized, and distributed) pharmacodynamics (biochemical and physiological effects of the drug), bioavailability and half-life (how long the medication is present in the body).

Phase I-Testing of drugs on healthy volunteers for dose-ranging and to determine safety, tolerability, pharmacokinetics, and pharmacodynamics. The effect of food on the medication may be assessed. Typical number of participants is 10-100.

Phase II-Testing of drugs on patients with a disease to assess safety and efficacy (how well a drug works). Dose ranging assessments and therapeutic dose are evaluated. Adverse events are collected and evaluated. Typical number of participants is 100-300.

Phase III-Testing of drugs on patients with a disease state to assess effectiveness and safety. This stage helps determine a drug’s therapeutic effect. Adverse events are collected and evaluated. Typical number of participants is 1,000-2,000.

Phase IV-Post marketing surveillance (pharmacovigilance-monitoring after the drug is approved for use). This stage is conducted to assess the long-term effects of a medication and can also be conducted in special populations (pediatric, pregnancy, etc.). These studies may be required by regulatory agencies to further evaluate the profile of the medication approved.


Terry L. Anderson, PhD

Field Medical Director

Pfizer, Inc.





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