Clinical trials for new drugs are conducted in a structured, multi-phase process. Phase I assesses safety and dosage in a small group of healthy volunteers. Phase II evaluates efficacy and side effects in a larger group of patients. Phase III confirms efficacy, monitors side effects, and compares it to standard treatments in a large patient population before regulatory submission. Each phase refines the risk benefit ratio of the treatment, and a trial can be stopped when that ratio becomes unfavorable.
