# Stages of Clinical Trials The traditional paradigm for drug development is divided into four stages: ## Phase I The purpose of a Phase I trial is to figure out what dose is safe to give to humans and by what mechanism to administer the drug. The sample sizes are usually small (relative to clinical trials). ## Phase II The purpose of a Phase II trial is to establish that a drug actually has benefit in a patient population and to estimate initial treatment effects. They can also be used to figure out if drugs have side effects or toxicity. Drugs that show promise make it to the next stage. The sample sizes are usually moderate (50-300 people) with a moderate study duration (months to a few years). ## Phase III The purpose of a Phase III trial is to establish *comparative* effectiveness of a new treatment. "Comparative" means that it is being compared against a placebo or a current standard of care. A medicine worth investing in only if it's demonstrably better than what we have currently, controlling for all other factors. It can also be used to collect long-term toxicity data. The sample sizes are usually quite large (hundreds to thousands), and they can last several years. ## Phase IV The purpose of Phase IV trial is to continue monitoring a drug's effectiveness and safety in the long term. They can also be used to study different doses or populations that were not originally considered in preceding trials. Phase IV trials also often leverage [[Real World Evidence (RWE)|real-world evidence]] as additional evidence for these metrics. ## Exceptions There are novel clinical trial designs that blur the line between these phases. For example, a seamless phase II/III trial quickly establishes efficacy and then shifts to studying comparative effectiveness. --- # References [[Introduction to Statistical Methods for Clinical Trials#1. Introduction to Clinical Trials]]