From the Super Mario Wiki
Negative Ions are a liquid-type spray of ions used in Mario & Luigi: Bowser's Inside Story. 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.