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, Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey. Being the equivalent of Mushroom Tea and Emoglobin Sweat that Mario and Luigi use, Negative Ions fully restore 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 to activate it. Each spray costs ten coins to use. 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.