Prophetic experiences often involve symbolic actions that illustrate the future of various nations. The primary approach among commentators is that the events described do not take place in physical reality. Rather, the entire sequence happens strictly within a prophetic vision.
In this vision, the prophet takes a cup. This action represents the spiritual act of receiving a prophecy directly from God. Giving the poisonous cup to the nations to drink is also symbolic. It represents the prophet delivering God's harsh and difficult messages to the nations he was sent to address.