Phenibut is a central nervous system depressant with anxiolytic effects, and is used to treat anxiety, insomnia, and a variety of other indications. It is often used "off label" for social anxiety. Phenibut is structurally related to the neurotransmitter γ-aminobutyric acid (GABA), and hence is a GABA analogue. Specifically, phenibut is GABA with a phenyl ring substituted in at the β-position, and thus can be written as β-phenyl-γ-aminobutyric acid (or, shorthand, β-phenyl-GABA). It is this phenyl ring substitution that allows phenibut to cross the blood–brain barrier, unlike regular GABA, which acts more peripherally. At low concentrations, phenibut can mildly increase the concentration of dopamine in the brain, providing stimulatory effects in addition to the anxiolysis. This makes sense because phenibut also contains the trace amine β-phenethylamine in its structure.
Unlike benzodiazepines, phenibut acts as a full agonist of the GABAB receptor.  At higher doses, phenibut loses its selectivity of GABAB, and gains additional activity as a GABAA agonist. It is these effects at the GABAB receptor that are responsible for Phenibut's sedating effects.
Contraindications of phenibut include:
IF—and only if—you are not contraindicated, Phenibut may be taken in divided doses of up to 3g/day. Most users will need far less than this, something on the order of 500mg (1 capsule) to 1000mg (2 capsules) per dose or per day. Phenibut shouldn't be taken on consecutive days in perpetuity.
Drug-Drug Interactions (DDIs): Phenibut may mutually potentiate and extend the duration of the effects of other central nervous system depressants including anxiolytics, antipsychotics, sedatives, opioids, anticonvulsants, and alcohol. Ask your doctor before combining phenibut with any other prescription medication.