The hot-topic of the vitamins

so this is just my speculation but , I think that the 'vitamins' are some form of addictive medication like opiods or CNS depressents (sedative + causes change in personality and memory issues) . Note that side effects to drugs like these would be very mild and furthermore, Hwon gives them to woojin before sleeping so you wouldnt notice an 'effect' as such. I think Hwon used these so that the effects of the drug would make Woojin's body physically drawn to him through subtle addiction. This sort of psychology is used way more than you think. Ill explain how it works briefly wih a simpler explanation; for an example lets say you bring a chocolate to a chocolate lover everytime you saw them without mentioning anything, just acting as if its normal everyday stuff. After a while, they'd then begin to subconsiously relate your existence with feeling nice and enjoyment so that everytime they seen you, BAM instantly happier even before they recieve the expected chocolate. Without realising, they become dependent on you (the giver) for that form of enjoyment or happiness. Hwon's 'vitamins' worked exactly the same way on Woojin. Woojin changed completely and became clingy and doting upon Hwon. When they were separated for long periods of time, Woojin became aggitated, restless and needy, he memory even begun to play up. ; he became dependent of Hwon for his happiness. This is just my theory, I think the story makes a bit more sense with it.