Phenibut: general

Phenibut is a structural GABA analogue and acts as an agonist [1]. Structurally it is related to baclofen and gabapentin, two other GABA-analogue drugs [2]. Exactly which receptors drive its effects is disputed: some sources describe it as acting mainly at GABA(B) [3], others as GABA(B) with some GABA(A) involvement [4], others add a role for β-phenethylamine receptors [1] — this is not settled.

It was developed in the Soviet Union in the 1960s, originally for military use [1], and later moved into clinical medicine [1], reportedly including a place in the Russian cosmonaut kit as an anti-anxiety medication [5]. It is still available by prescription in Russia today, used there for anxiety, PTSD, stuttering, and vestibular disorders [5], and it remains a registered pharmaceutical in some other countries while also being sold online as a sleep aid, mood enhancer, anxiety treatment, and cognitive enhancer [2]. It's found as tablets, powder, or crystalline material [2].

Its effects are anxiolytic and nootropic — it reduces anxiety and is reported to affect cognition [3] — and it is reported to act as a central nervous system depressant [6].

The one thing that matters most before anything else: phenibut is reported to act as a central nervous system depressant [6], and as with depressants generally, combining multiple sedating drugs — including alcohol, benzodiazepines, or opioids — substantially increases the risk of overdose and fatal respiratory depression [7]. Don't stack it with other sedatives, don't use it alone, and have a plan for emergencies.

References

1. Phenibut: A drug with one too many “buts” — Wiley (Basic & Clinical Pharmacology & Toxicology)
2. Phenibut - Expert Committee on Drug Dependence Information Repository — World Health Organization (WHO) ECDD
3. PHENIBUT — NIH National Center for Advancing Translational Sciences (NCATS) Inxight Drugs
4. Phenibut (beta-phenyl-GABA): a tranquilizer and nootropic drug. — Europe PMC (European Journal of Pharmacology, 2001)
5. Phenibut Use and Withdrawal — University of Virginia School of Medicine, Division of Medical Toxicology
6. Phenibut — Wikipedia
7. Anxiolytics and Sedative-Hypnotics Toxicity - StatPearls — NCBI Bookshelf (US National Library of Medicine)