Final answer:
The last phase of human medication testing is Phase IV. This phase involves post-marketing surveillance to gather additional information about the drug's safety, efficacy, and long-term effects.
Step-by-step explanation:
The last phase of human medication testing is Phase IV.
In Phase IV, the drug has already been approved by the regulatory authorities and is available on the market. The purpose of Phase IV trials is to gather additional information about the drug's safety, efficacy, and long-term effects in a larger population. These trials are typically conducted after the drug has been approved and are also known as post-marketing surveillance.