Can chewing on one side of the mouth be harmful?

Yes.

By chewing only on one side, only the muscles of the one side of the face are accessed and used. Over time, with the activation of only one side of the face, this can cause a facial asymmetry.  Further, this can have negative impacts on the bite and the TMJ.  The bite can be altered from increased use of teeth on one side of the mouth and the TMJ (temporomandibular joint) may suffer from increased use.  With overuse of muscles on one side of the face, you can often see a visual difference when comparing the right and left side of the face.  

If your child, or yourself is chewing on only one side of the mouth, it is time to start asking why.  Our body has many tells that are built in but can be easily missed or misinterpreted.  This may be from poor dental care, lack of tongue function to create a bolus, or from a previous trauma.