Use a hidden item. Takes time, but no item are expended.

In-Game "Help" Description.

Stash (とっておき, Totteoki?, lit. Prized Possession) is a skillset used by the Alchemist dressphere in Final Fantasy X-2 that allows the user to use items irrespective of the player's inventory. Stash abilities have a longer charge time than the Item command, and they do not benefit from the Chemist or Items Lv.2 effects, but can be used infinitely once learned.

Captured creatures can use the Stash version of Mega-Potion when equipped with the Alchemist dressphere. Likewise, the following captured creatures also have access to various Stash abilities:

• Goon: Phoenix Down, Elixir
• Yaibal, Gippal, Buddy: Mega-Potion (can use it even when not using the Alchemist dressphere)

List of abilities

Name	AP	Description	Prerequisite	Image
Potion	10	Use a spare Potion that you tucked away.	None
Hi-Potion	40	Use a spare Hi-Potion that you tucked away.	Potion
Mega-Potion	120	Use a spare Mega Potion that you tucked away.	Hi-Potion
X-Potion	160	Use a spare X-Potion that you tucked away.	Mega-Potion
Remedy	20	Use a spare Remedy that you tucked away.	None
Dispel Tonic	20	Use a spare Dispel Tonic that you tucked away.	Remedy
Phoenix Down	30	Use a spare Phoenix Down that you tucked away.	None
Mega Phoenix	200	'Use a spare Mega Phoenix that you tucked away.	Phoenix Down
Ether	400	Use a spare Ether that you tucked away.	Dispel Tonic
Elixir	999	Use a spare Elixir that you tucked away.	Ether