Board Thread:Manga/@comment-5774380-20160408232143/@comment-24407809-20160414195919

Manuel de la Fuente wrote:

Tausendberg wrote:

That is the question, what effect, if any, would titan serum have on a titan shifter? I speculate none perhaps but who knows? In Eren's case, the serum he got injected in the Reiss Chapel made him develop his hardening abilities; but we don't know if that serum and Levi's serum are of the same class. Maybe the serum Eren received was only meant to develop extra abilities and not a specific Titan form.

My guess is that if you are already a Titan Shifter and get injected with a second serum designed to create another Titan form, there would be a conflict and you would lose control of your forms. Eren didn't inject himself with anything, he just ate that vial of "Armor" (not sure how he knew that injesting it would do anything though).