Phenibut, is a very sour tasting, water soluble derivative of gamma aminobutyric acid (GABA) (as seen below). To be specific, GABA + phenyl group, which is something that actually aids in crossing the blood brain barrier (BBB).
Phenibut is most well known for its anxiolytic effects, and from a physiology standpoint, it makes sense since most anxiolytic drugs (benzodiazapines/barbituates/alcohol) exert their effects upon GABA, in this case specifically GABA(B) more than GABA(A) and is most similar in action to baclofen.
Phenibut was developed in russia in the 1960s, where even today most of the research about it is originating from.
Phenibut has been shown to nootropic properties, in one rat study in particular it was shown to accelerate the acquisition of internal inhibition, to decrease and stabilize the time of inter signal reactions, increase heart rate and decrease respiratory rate (Ziablintseva et al). With all this said it becomes pretty obvious that phenibut should not be skipped over when compiling a body of nootropic knowledge and is something we are looking forward to knowing more about, specifically with regards to any potentiating effects it may be discovered to have along side other substances; not to mention seeing some research come out of some countries other than russia.