GHB, an GABA- b and GHB receptor agonist works within second or at least within minutes. Although the reboud is oposite ( dopamine related rebound? ). Something Phenibut lacks.
But the effects are not even alike. GHB is very Euphoric and short lasting. Phenibut is besides being a goog way to get of GHB/ GBL or Baclofen for that matter . Same mechanisme. But both these substances don't agree with me, especially Pheninibut with which I have exp with. It made me nausseaus even at minimal dosages and has an long acting depressing effect, on me, opposed to GHB. They share the naussau when overdone, but I would rather overdue GHB as its at least euphoric.