Phenibut

4-amino-3-phenylbutanoic acid

Overview

Phenibut belongs to Depressants.

Key safety note: Phenibut is widely sold online as a 'nootropic' or calming supplement, but it carries a serious risk of tolerance, dependence and a severe withdrawal syndrome that can include rebound anxiety, insomnia, agitation, tremor, hallucinations and, in some reports, psychosis or seizures: withdrawal has required hospital management.[3][2][6]
Effects
Subjective effects vary. What a substance feels like depends on dose, individual physiology, mindset, and setting. The points below describe commonly reported effects, not guaranteed, uniform, or desirable outcomes.
  • Anxiolysis, relaxation, sociability, mild euphoria and improved sleep are the sought-after effects. At higher doses, sedation, disinhibition and an alcohol- or baclofen-like intoxication.
  • The slow onset (often 2–4 hours) frequently leads users to redose prematurely, escalating the total dose and the risk of over-sedation.
  • Tolerance develops rapidly and regular use leads to dependence. Acute overdose, usually in combination with other depressants, causes marked sedation, reduced consciousness and sometimes agitation or delirium.
Dosing & duration
Harm-reduction note: These are commonly cited reference ranges, not a recommendation or a “safe” dose. Potency, purity, body chemistry, tolerance, and drug combinations vary widely. Start low, go slow, wait for full effects before redosing, and never assume an unknown product matches these figures. Missing data is not evidence of safety.

Oral (powder or capsules, usually the hydrochloride salt). Occasionally rectal.. The figures below are commonly cited reference ranges, not a recommendation or a 'safe' dose. Two features make phenibut particularly risky: its onset is slow (2–4 hours), which tempts premature redosing and accidental overdose, and it produces rapid tolerance and a severe physical dependence with a dangerous withdrawal syndrome (rebound anxiety, insomnia, agitation, tremor and, in some cases, hallucinations, psychosis or seizures). Combining phenibut with alcohol, benzodiazepines, opioids or other CNS depressants adds sedation and respiratory-depression risk. Do not use daily. After regular use, taper rather than stopping abruptly.

Dose ranges

Threshold

0.25 g

Light

0.5–1 g

Common

1–2 g

Strong

2–3.5 g

Heavy

3.5 g +

Duration

onset

1.5–4 hours (slow)

total

10–24 hours

after effects

6–24 hours. Rapid tolerance, and with regular use dependence and a severe withdrawal syndrome

Chemical & Physical Properties
FormulaC10H13NO2
Molar mass179.22 g/mol
StateSolid (usually the hydrochloride salt, white crystalline powder, zwitterionic amino acid)
Melting point≈253 °C (hydrochloride)
Boiling pointNot reported
DensityNot reported
Vapor pressureNot reported
pKaNot reported
LogP-1.6 (predicted, XLogP3, free base)
SolubilityNot reported
Refractive indexNot reported
Identifiers & Synonyms
CAS1078-21-3
CAS (enantiomer)
PubChem CID14113
InChIKeyDAFOCGYVTAOKAJ-UHFFFAOYSA-N
InChIInChI=1S/C10H13NO2/c11-7-9(6-10(12)13)8-4-2-1-3-5-8/h1-5,9H,6-7,11H2,(H,12,13)
SMILESC1=CC=C(C=C1)C(CC(=O)O)CN

Synonyms

  • Anvifen
  • Noofen
  • Fenibut
  • β-phenyl-GABA
Pharmacodynamics & Biochemistry

Phenibut (β-phenyl-GABA) is a phenylated analogue of the neurotransmitter GABA, developed in the Soviet Union in the 1960s and used there as an anxiolytic and nootropic. Its clinically relevant activity resides almost entirely in the R-enantiomer. Its principal mechanism is agonism at GABA-B receptors, acting as a weaker, longer-acting relative of baclofen, which produces its anxiolytic, sedative and muscle-relaxant effects. The S-enantiomer does not bind GABA-B. Additional GABA-A and dopaminergic effects have been reported at higher doses. Phenibut also behaves as a gabapentinoid: R-phenibut binds the α2δ subunit of voltage-dependent calcium channels: in rat-brain membranes with an affinity (Ki ≈ 23 µM) about four times higher than its affinity for the GABA-B receptor, which contributes gabapentin-like anti-nociceptive effects. Both affinities are weak (micromolar), which is why gram-range doses are needed for an effect. Its pharmacokinetics shape its use pattern and risks: a slow oral onset (2–4 hours), a long duration (up to about 24 hours), minimal metabolism and largely unchanged renal excretion.

Biological targets

  • GABA-B
  • VGCC α2δ
Pharmacokinetics
BioavailabilityOral ≥63% (250 mg dose)
TmaxNot reported
Half-life≈5.3 h (250 mg)
VdNot reported
Protein bindingNot reported
MetabolismMinimal hepatic metabolism
ExcretionRenal, ~63% excreted unchanged in urine
Toxicology & Safety
Harm-reduction note: Toxicity and risk depend on dose, route, purity, combinations, setting, and individual health factors. Missing harms should never be interpreted as evidence of safety.

Not reported

Phenibut is widely sold online as a 'nootropic' or calming supplement, but it carries a serious risk of tolerance, dependence and a severe withdrawal syndrome that can include rebound anxiety, insomnia, agitation, tremor, hallucinations and, in some reports, psychosis or seizures: withdrawal has required hospital management. Its slow onset (2–4 hours) encourages premature redosing and accidental overdose. Acute toxicity, usually in combination with alcohol or other depressants, presents as heavy sedation, reduced consciousness and sometimes agitation or delirium. It should not be used daily, and after regular use it must be tapered rather than stopped suddenly. It is an unapproved drug in most Western countries and is banned outright in Australia.[3][2][6]

Legal Status
Legal note: Legal status can change over time and may vary by country, region, formulation, analogue status, prescription context, and enforcement practice. Always confirm with current official sources before relying on this section.
Interactions & Contraindications

Drug interactions

Alcohol, Benzodiazepines, Opioids Additive sedation and respiratory depression: the main acute danger.[3]
Baclofen and other GABA-B agonists Baclofen and other GABA-B agonists add cross-tolerant CNS depression.[2]

Contraindications

Known hypersensitivity to the drug hypersensitivity to phenibut.[2]
History of stimulant or substance use disorder history of substance-use disorder or dependence (high dependence and misuse potential).[3]
Combining with alcohol or other CNS depressants concurrent alcohol, benzodiazepines, opioids or other CNS depressants.[3]
Abrupt discontinuation after regular use (withdrawal and seizure risk) regular or daily use, and abrupt discontinuation after regular use (severe withdrawal, seizure risk), so taper.[3]
Pregnancy or breastfeeding safety not established.[2]
Usage & Context
  • A prescription anxiolytic and nootropic in Russia and some post-Soviet states (as Noofen, Anvifen and Fenibut) for anxiety, insomnia, tension and related conditions. It is not an approved medicine in most Western countries.
  • Widely sold online as a grey-market 'nootropic' or relaxation supplement and used recreationally for its anxiolytic, mildly euphoric, alcohol-like effects: a pattern strongly associated with tolerance, dependence and a difficult withdrawal.
Sources & Evidence

Further Information