What stage ensures the drug’s ongoing safety after it reaches the market?

The phase that ensures a drug's ongoing safety after it reaches the market is Phase 4. This phase, also known as post-marketing surveillance, is critical for monitoring the long-term effects and safety of a drug once it is available to the public.

In Phase 4, the drug is already approved for use, and health authorities and the drug manufacturers collect data on the drug's performance in the general population. This phase helps identify any rare or long-term adverse effects that may not have been evident during earlier phases of testing, which primarily focus on efficacy and short-term safety in a controlled environment.

The information gathered during Phase 4 can lead to additional labeling changes, the issuance of warnings, or even the withdrawal of the drug from the market if significant safety issues are identified. Thus, it plays a crucial role in ensuring that any potential risks associated with a drug are identified and managed effectively post-approval.