Rei being held together by medication was never really established in the anime, so I wouldn't hold to it as canon across the whole franchise. It's possible that Rei was taking pain pills for her pre-episode 1 injuries, but it looked to me like there was too much there just to be pain meds, and also there was the fresh looking RX bag full of unused pills. However, I think it's quite possible that the purpose of Rei's medication is not necessarily a physical one, but a psychological one.
In the alternate reality of episode 26 (the "Angelic Days-esque" scenario), the Rei we saw was not the quiet introvert that we knew from the rest of the series, but rather a somewhat hyperactive but healthy teenage girl. I know that it's an alternate reality so it shouldn't be taken too seriously, but what if the reason for Rei's unusual behavior is because this is Rei un-medicated?
Think about it-- if Gendo wanted Rei to co-operate with his design for the apocalypse, if it were the Rei from e26 it would be pretty friggin' difficult. But if he keeps her in a zombied-out state through medication, she would probably take on what Gendo would consider a much more "agreeable" personality. Given the psychological nature of Evangelion's story, it would be pretty fitting to have an element of the story that deals with the use (or in Rei's case, I would argue misuse) of personality altering medications.
This subject is of some personal interest to me, since as a child I was diagnosed with ADD, and one doctor recomended treating it with Ritalin (to which thankfully, my mom politely said "up yours"). If Rei's medication was the primary reason for her rather vacant personality, then it only makes her story all the more tragic, and Gendo and even more evil bastard. What could have been an otherwise healthy, happy teenage girl ended up being turned into the doped-up puppet of a crazy SOB who wants to start the apocalypse.
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