Yes its possible to over dose on xanax like any other medication. xanax is a benzo or a nervous system deppressant. Too much of it will cause your respitory system to shut down. I think what your asking is "if the withdrawl cuased her to commit suicide?" Yes, it could be a possiblity since xanax withdrawl can lead really bad phyiscal symtoms such a seizures or death or mental symtoms like panic attacks, anxiety depression, emotional unstability and suicidale thougths..I know this cause i went through it..but yea it just as bad as meth or heroin if not worst...
It is very possible to overdose on Xanax. You say it was "presumed" she OD'd on her surplus Xanax. Was an autopsy done? I certainly don't want to come across as a "champion for Xanax," but I think you need more information before you lay this sad and very tragic death at the feet of Xanax.
I am very sorry for your loss.
Peace
Greenlydia