Phase IV trials are primarily conducted to evaluate safety after marketing in a large population.

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Phase IV trials focus on post-marketing surveillance to monitor safety and effectiveness in a broad, real-world population. After a drug is approved, these studies look for rare or long-term adverse effects that may not appear in earlier, more controlled trials, and they help refine safety information, inform labeling, and guide ongoing risk management. This is why post-marketing safety studies in a large population is the best description of Phase IV purpose. In contrast, pre-marketing efficacy assessments occur earlier to establish whether the drug works (Phase III), early toxicity screening in healthy volunteers is Phase I, and cost analyses after marketing fall under pharmacoeconomic evaluations rather than a clinical trial phase.