User blog comment:KiruSama/Levi and the Titan Serum/@comment-24930713-20151120002210

I feel if he used it on another Titan Shifter it would have no effect. I mean, the whole point of the serum is to turn a human into a Titan, but they've already passed that phase so I can't imagine it having any effect on them. As for Levi being a Titan, that's not really something I look forward too. Levi is my favorite character, and I feel with all his skills, Isayama has been trying to find legitimate and beleivable reasons to keep him out of certain fights so that the story progresses as intended. So I highly doubt he'd make humanity's strongest soldier into a Titan. There's also the possibility that as soon as he becomes one, it would give the Beast Titan the upperhand.