The Casimir effect is a remarkable macroscopic feature of QED, while recent lattice studies have also shown its potential nontrivial consequences in QCD. In light of having a better understanding of the Casimir effect, it is advantageous to have a self-contained path integral formulation of the phenomenon. I will show how the Casimir effect between two uncharged plates in the presence of a chiral medium, modeled with an axion term θF~μνFμν, can be formulated in terms of the path integral, and how such a formulation leads to a 3D effective action of the restricted electromagnetic field.