Board Thread:Manga/@comment-5774380-20160408232143/@comment-5774380-20160414194703

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.