Drug
From Starfy Wiki
The Drug (Japanese: クスリ) is an item in Densetsu no Starfy, found in the Deep Sea in the door between a waterfall. It is a potion full of a red liquid. It is not stored on the Pause Screen.
Obtaining this item is optional. Gekojii gives it to Starfy as part of an experiment to see if it will increase the power of the Star Spin and Starfy presumably drinks it, making Starfy learn the Ultra Star Spin. Starfy must break all blocks in a series of four rooms within a time limit of 90 seconds. If Starfy fails, he will lose one heart and the player will have to try again.