Gabapentin isn't cross-tolerant with opiates so no need to worry about that. IMO gabapentin is one of the best remedies for opiate withdrawal that isn't cross-tolerant with opiates. There are alot of meds useful for opiate withdrawal, but is seems that they're either cross-tolerant (kratom and loperamide for exampe) or just mask the symptoms with their primary effects (like benzos and cannabis), but few actually downright lessen the severity of withdrawal by action seperate of the "high" while also not hindering withdrawal progress.
I typically use higher doses than 600mg, though that's definitely a good place to start. Just keep in mind that gabapentin can take up to a few hours to kick in and although it can provide a very nice buzz, it's pretty mild so don't expect it to get really high. My doses range from 900mgs all the way up to 2,400mgs, but that's because tolerance rises pretty fast if your taking large doses. It should go without saying that you shouldn't start with doses on the latter end of that scale.
By the third day it becomes solely withdrawal relief and pretty much no high, in my experience at least. Anyways, I guess I just wanted to vouch for it's effectiveness as a withdrawal remedy... it's something I'll never kick without. Lyrica is obviously better, but it's incredibly hard to obtain unless you have neuropathic pain or know someone who does. Gabapentin on the other hand can be purchased easily and legally through the web. Can't say if you'll find gabapentin as useful as I do though.
