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Photo- Andres Kudacki.
I think we can pretty easily figure out that this is likely an energy boosting supplement.
Points to consider are: Taken in a public place at an event he would have known about prior. Blue in color, staining teeth, which implies its being chewed. Rectangular shape. Trump famously being straight-edge. (No alcohol, or anything.)
So it's something he's taking as needed. If it was a scheduled drug he would take it in private. It's unlikely to be illicit like meth, or something. And the form narrows it down a lot too. For example there are ~~chewable~~ nitroglycerin tablets that you might take as needed for chest pain, but theyre not blue or chewed. Since I don't think he's ever smoked nicotine tabs are off the table too.
So it's probably just an energy booster. Like B-12, or Caffeine. It's common among older people to take stuff like this to stay alert throughout the day. Which some can have blue dye in them which explains the color, and teeth being stained.
Edit: I just realized i said the nitroglycerin are both chewable and not chewed. I meant nicotine is chewable, and nitorglycerin isnt lol