Phenibut is a bit of a GABA analogue, designed by russsian scientists to be a middle ground betweeen the actual muscle building GABA and phenazepam (a benzodiazepin, much like Valium).
Don't read too many horror stories about it, if you're already a bit of a clonazepam junkie, Phenibut will do pretty much nothing unless taken in huge quantities, over 2mg per pop, or even more (not recommended).
Now on a body that has not ever been 'contaminated' with benzos,valerians and serotonin stufff, it might be efective even at 250mg.
But don't expect anything miraculous. Just a slight decrease in t volume and a bit of sedation, that's about it. Just a lot more effective than plain GABA but a lot less effective than clonazepam. That's why it's legal, probably.