phenibut

Developed and widely used in Russia, phenibut is an analogue of GABA with a phenyl ring substituted at the $\beta$ carbon, giving it the name $\beta$-phenyl-$\gamma$-aminobutyric acid or $\beta$-phenyl-GABA:

Recreationally, it's interesting in that it seems to provide many of the social benefits of alcohol without the drawbacks of mental fuzziness and impaired sleep.

Nick Cammarata says: "Phenibut is a generally magic and wonderful drug that’s very close to a free lunch if you’re super careful — don’t mix with other drugs, don’t raise dose, and don’t do often — and terrible if you don’t care about safety" (thread).

Effects: as a GABA receptor agonistFrom Psychiatry Online: "It acts mainly as a GABA-B receptor agonist, with some activity at the GABA-A receptor, in addition to stimulating dopamine receptors and antagonizing β-phenethylamine (endogenous anxiogenic)"., phenibut is (like ethanol and benzodiazepines) a depressant, with reported effects including reduced anxiety and inhibitions, euphoria, pro-social feelings (it is a mild entactogen), muscle relaxation, increased libido, and stimulation or sedation depending on the dose. It differs from benzodiazepines by acting mostly at GABA_B receptors, while benzos act primarily on GABA_A receptors.

Negative/side effects can include: nausea, headaches, respiratory depression, loss of motor control, increased blood pressure.

As with alcohol and benzos, phenibut dependence is a thing and withdrawal is apparently very unpleasant. Phenibut dependence is rapid: just 2-3 consecutive days of use can be enough for severe withdrawal symptoms.

Using: on days when I take phenibut I usually take 600mg orally in the morning, sometimes supplemented with another 300mg a few hours later (for 900mg total). There are reports on Erowid of higher doses (from 1-2g up to 5g and more) being taken without acute harm, but with greater side effects and often causing extended sleepiness.

Phenibut has a slow onset of action, up to 2-4 hours, so that people are often tempted to redose; don't do this. The half-life in the body is reportedly 5.3 hours, so a dose lasts quite a while (longer than alcohol).

It should not be taken regularly due to the potential for dependence. "I’d be scared taking it more than once a week. I feel pretty safe at biweekly" (link). Personally I take 'once per week' as my guideline with four days as the absolute minimum spacing. For example, if I take phenibut on Sunday I could take it again on Thursday, formally allocating these doses to different 'weeks', but this can't be a sustained pace; over any multi-week span I should average at most one dose per week.

In my experience it can be okay to combine phenibut with alcohol in small quantities: treat each drink as if it were 3 drinks, and be exceedingly careful. It's much easier to overdose on alcohol while on phenibut, leading to sedation, vomiting, respiratory depression and even death.

References:

• Psychonaut Wiki
• CNS Drug Reviews: Lapin, I. Phenibut: A Tranquilizer and Nootropic Drug (2001)