Board Thread:Questions and Answers/@comment-25322628-20160519191537/@comment-25322628-20160519233058

GodKingReiss wrote: As far as we know, anyone can use a Titan serum, maybe even people who are already shifters (Eren injesting the "armor" bottle). Eren ingesting the armor version could stem from already being a shifter.

But if anyone can be turned into a Titan through the serum, it would be interesting to see a chapter where the antagonist shifters are turned this way and we know how it was done for them.