They don't know what that is exactly, but it's definitely some kind of medicine--they slip forward and snatch the bottle up--and oh, wow, that is blood on his lips--

"Can you--can you--make more of this stuff?" Frisk waves the bottle in one hand, waves their bag of chips in the general direction of Goku in the other, even if they realize that might not be the best idea. Getting medicine wrong can make things worse than sickness can. Though coughing up blood is certainly pretty bad on its own.