Like canonically? Obvs in-game they level you up, but from what little I remember, levels weren't a thing in the show, at least not after a while. They went the way of the DBZ Scouter (right?) Not to mention it's a nonsensical concept as it is. Quantifying 'power' would necessitate quite a few details that the writers are clearly not interested in even thinking about.

All this is to ask if Rare Candy are just roids. Are they steroids? Pokeroids? Is there Pokemon roid rage? If I feed my weavile too many, will she not only get really fat but be wired as fuck and angry, and want to blow off steam by riding my face all the time? Asking for friend. This sounds like a Dr. House episode, where someone's zangoose girlfriend is losing her shit more and more. Just aggressively horny all the time, and eventually you go to the doctor because you both assume it's some kind of brain tumor. Turns out she bought candy off of Pokemon Temu and it's got budget Pokeroids in it.