There have been suggestions in the literature that lithium depletes inositol in the brain, leading to side effects.

This article clears that up some, as real evidence has been scanty on this action in the past.

http://www.nature.com/npp/journal/v1.../1395201a.html

Myo-inositol mentioned in this article is the inositol used as a supplement and modified by the body to myo form.

Yes, when the body accommodates to a supplement by changing the chemistry --upticking it etc---then the perceived benefits of that supplement tend to wane and weaken. This may be prevented by taking some supplement free holidays so that this is less likely to happen. Then the patient would sense a "wearing off" of effect.

I do not think the PharmaGaba would be the same as the inositol with the lithium...but this has not really been studied.

This is a long article, and you can skip around to read the discussions etc. You may have to read it more than once.
Near the end is usually where the explanations occur...so that is where you should pay the most careful attention.

Inositol in mania:
http://www.ncbi.nlm.nih.gov/pubmed/16542185

So because there are papers like the one just immediately above, out there, be careful with inositol.