Select an anesthetic agent to administer from this drop-down.

You can only select or change the agents to administer at the start of a simulation. However, you can always change the vaporizer settings for any agent later on in the simulation, so you can (realistically) switch anesthetics during an operation by turning down the vaporizer for the first anesthetic and turning up the second, provided that both agents were specified at the start (see Multi-agent Simulations).

You can also context click (right click) this table on the row for an anesthetic, which will display a menu you can use to perform several functions in the context of that anesthetic. (see Agent Context Menu).