Clinical trial phases
What each stage of clinical testing is actually designed to answer.
Phase I asks primarily about safety and dose in a small group. Phase II looks for a signal of efficacy and refines dosing in a larger one. Phase III tests efficacy against a comparator in a population large enough to detect meaningful differences and uncommon harms — this is where most regulatory decisions are made. Phase IV follows the intervention in routine use, where rare effects and long-term outcomes surface.
The distinction that matters most to readers is what a phase can support. A Phase II result is a reason to run Phase III, not a reason to believe a therapy works; encouraging Phase II findings frequently fail to replicate. Randomisation, blinding, and a pre-specified primary endpoint are what make a trial informative — and endpoint choice is where surrogates can quietly substitute for what matters.
This is why the incretin outcome trials carried such weight: they measured events directly. For regenerative products, blinding is often impossible — a surgical implant has no easy placebo — which is a real methodological constraint, not an oversight.
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