User blog comment:Xilinoc/Ch. 552 - The Fundamental Virulence/@comment-5695542-20131008224031/@comment-11051098-20131009152134

It's been stated that the owner of the medallion that stole the Bankai, can use it by will, even after the death of the Shinigami who had the Bankai.

I suspect that once the Shinigami consumed the pill and the Bankai was slightly affected by Hollow-powers, the body of the Quincy couldn't stand wielding it. I think that's how and why Cang Du was injured the second the pill took effect.