The concept for a clinical trial starts with scientific research in a laboratory, where a potential new treatment is tested in cell and animal models. The most promising potential new therapies then move into clinical trials and are tested in humans.

The possible new treatment moves through a series of steps or phases of clinical trials to gain more knowledge about the safety and effectiveness of the therapy.

  • Phase I trials – A new drug or treatment is tested for the first time in a small group of healthy people (20-80). Phase I trials test safety, determine the right dose, and identify any possible side-effects.
  • Phase II trials – A new drug or treatment is given to a larger group of people including patients (100-300) to further study safety and to determine efficacy.
  • Phase III trials – The new drug or treatment is given to an even larger group of patients to compare investigative drug or treatment to the standard of care and to further confirm effectiveness and safety. If results are positive at this phase, the new drug or treatment will then be approved by the Food and Drug Administration (FDA) and available to patients.
  • Phase IV trials – After a drug is approved by the FDA and available to the public, additional data is collected at this phase to track safety in the general public and to monitor treatment benefits and optimal use.

The process of drug development can be lengthy. The timing and success rate of candidate drugs through the different phases of clinical trials varies: (

  • Phase 1 – length of study lasts several months, approximately 70% of drugs move to next phase.
  • Phase 2 – length can be several months to 2 years, approximately 33% of drugs move to next phase.
  • Phase 3 – length ranges from 1 to 4 years, approximately 25-30% of drugs move to FDA approval.

Pulmonologist Roham Zamanian discusses the complex, time-consuming and costly process of developing safe, effective drugs. Learn about the full lifecycle of drug development, from inception in the laboratory through clinical trials to FDA approval.

Daniel C. Grinnan, M.D., explains the role of the U.S. Food and Drug Administration (FDA) in drug approval in this 11 1/2-minute PHA Classroom video.

This infographic shows what the drug development process looks like (click to enlarge).

Learn More About How Clinical Trials Work

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