They are considered to be the same when calculating max dosage.
The relationship is 300mg Lyrica = 1800mg Neurontin.
They seem to be in the same family and do the same type of thing,
and are to be considered the same med when titrating from one to the other,
so as not to exceed the recommended dosages.
Combining them or taking them together is thought to be overkill,
(or overdosage problems).
I don't understand why it would be done.
That said, I've read quite a few posts on other forums, that said the docs have
Rx'd them (or so they claim) to be taken at different times of day ex: Lyrica 3xday & Neurontin before bed. Or use of Neurontin as breakthru med (that doesn't make any sense to me, as it takes about an hour to kick-in).
I think that any affect that additional Neurontin over Lyrica is purely psychological.
It makes no sense, since Lyrica is the more effective, stronger of the two similar meds.
In regard to the dosage where it becomes ineffective, that would be on an individual basis, and differ from person to person. My tolerance was at 4200mgs/day after over 6 yrs using the med in increasing daily amounts. After 2yrs at 4200mgs.day, it started to become ineffective & I switched to Lyrica. 600mgs/day after titrating up.