From the Super Mario Wiki
Negative Ions are a liquid-type spray of ions used in Mario & Luigi: Bowser's Inside Story and its 3DS remake. Being the equivalent of Mushroom Tea and Emoglobin Sweat that Mario and Luigi use, it fully restores Bowser's HP and SP. For Bowser to obtain a blast of ion mist, he has to go to a Shop Block, managed by Broque Monsieur, and punch it. Each spray costs ten Coins. Bowser is able to use Negative Ions whenever he comes to a Shop Block, and it can still be used even if his stats are already maxed out.