Phase III is the final test before approval. These trials are large, often involving thousands of participants across multiple sites and countries. They are designed to confirm efficacy, monitor adverse reactions, and compare the new drug to the current standard of care. The results are the main basis for regulatory approval. A Phase III trial can take years and cost hundreds of millions of dollars. Failure at this stage is costly, but it happens. Roughly half of drugs that enter Phase III do not reach the market.

The design is usually randomized and double-blind. Participants are assigned to the new drug or a control, which may be placebo or an existing treatment. The primary endpoint is pre-specified and clinically meaningful, such as overall survival, stroke prevention, or hospitalization reduction. Secondary endpoints provide additional evidence. Data are monitored by an independent data safety monitoring board, which can recommend stopping the trial early if the drug is clearly effective or clearly harmful. Statistical analysis plans are written before the trial begins to prevent bias.

What Phase III establishes:

• Confirmatory efficacy. Does the drug work in a large, diverse population?
• Comparative effectiveness. How does it compare to existing treatments?
• Safety profile. What adverse reactions occur, and how common are they?
• Risk-benefit. Do the benefits outweigh the risks for the intended population?

After Phase III, the company submits a new drug application or biologics license application. Approval is not guaranteed.