The first time a drug is tested in humans—focused on safety and dosing, not whether it works.Phase I clinical trials evaluate safety, tolerability, pharmacokinetics, and pharmacodynamics in a small number of subjects (typically 20–80), usually healthy volunteers. Primary objectives are determining maximum tolerated dose (MTD), dose-limiting toxicities (DLTs), and pharmacokinetic parameters. Phase I does not assess efficacy. For peptide therapeutics, Phase I trials establish injection site tolerance, immunogenicity potential, and metabolic stability in humans. A compound reaching Phase I has completed preclinical toxicology and obtained an IND.