  Recent work by Philbin [1] has provided a Lagrangian theory that establishes
a general method for the canonical quantization of the electromagnetic field in
any dispersive, lossy, linear dielectric. Working from this theory, we extend
the Lagrangian description to reciprocal and non- reciprocal magnetoelectric
(bi-anisotropic) media, showing that some versions of the constitutive
relations are inconsistent with a real Lagrangian, and hence with quantization.
This amounts to a restriction on the magnitude of the magnetoelectric coupling.
Moreover from the point of view of quantization, moving media are shown to be
fundamentally different from stationary magnetoelectrics, despite the formal
similarity in the constitutive relations.
