I don't believe that bone effects have been proven yet for ALL drugs in the anti-seizure family.

This new article reiterates that:
http://www.ncbi.nlm.nih.gov/pubmed/17583832

This older paper from 2002 says that older drugs (enzyme inducers--refers to effects on the liver) have a higher incidence of bone effects than the new agents like gabapentin.

http://www.ncbi.nlm.nih.gov/pubmed/12011279

But I don't think there are definitive studies on bone loss with gabapentin. You might call the Pfizer hotline for patients and ask.

I think vitamin D status, magnesium status and B12 status also impact bone signficantly. Any lows with either of these 3 will also give lower density.

This Medscape article repeats that the newer agents like gabapentin have not been studied: (but it is from 2004)
http://www.medscape.com/viewarticle/537807

We have had people here using Lyrica claiming it rotted their teeth. Whether this is a calcium leaching thing or not, has not been proven yet.