F-Phenibut
4-fluorophenibut
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F-Phenibut (4-fluorophenibut) is a synthetic depressant of the β-aryl-GABA class. It is a fluorinated structural analog of phenibut, distinguished by roughly 58-fold greater potency.[1] F-phenibut is a GABA-B receptor agonist, producing its effects by enhancing inhibitory signaling through G-protein-mediated suppression of neuronal excitability.[2]
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Synthesized by Ciba-Geigy as the developmental compound CGP-11130, F-phenibut was never advanced to clinical trials or approved for medical use in any jurisdiction.[3] The compound remained obscure until it appeared on online research chemical and nootropic markets in the 2010s. It circulates primarily through unregulated vendors and has been detected in Australian wastewater surveys confirming population-level use.[4]
Subjective effects include anxiolysis, sedation, cognitive euphoria, muscle relaxation, and sociability enhancement. The experience is described as a clean dissolution of anxiety into warm, heavy calm — similar in character to phenibut but compressed into far smaller doses, with a steeper climb from relaxation to incapacitating sedation. Motor impairment, nausea, and excessive drowsiness are common unsought effects.
F-phenibut produces rapid physical dependence, and withdrawal can include seizures, psychosis, and autonomic instability.[5][6] The 58-fold potency increase over phenibut creates substantial overdose risk when users miscalculate dose conversions — milligram-level errors produce qualitatively different levels of CNS depression.[1] No controlled human studies, pharmacokinetic data, or clinical trials exist; the entire evidence base rests on a single electrophysiology study, class inference from phenibut and baclofen, and user reports.
History[edit]
Discovery
F-phenibut was synthesized by Ciba-Geigy (now Novartis) as part of a systematic exploration of β-aryl-substituted GABA analogs, receiving the developmental code CGP-11130.[3] The exact year of synthesis has not been published. The compound sits between two better-known relatives — phenibut, introduced as an anxiolytic in the Soviet Union in the 1960s,[7] and baclofen, the para-chloro analog that became the only member of the series to achieve Western regulatory approval.
Market appearance
F-phenibut surfaced on online nootropic and research chemical markets in the 2010s under names including "fluorophenibut," "fluorobut," and "4F-phenibut." Its first peer-reviewed pharmacological characterization came in 2020, when Irie and colleagues used mouse cerebellar electrophysiology to establish its GABA-B potency — approximately 58 times greater than phenibut.[1]
Present circulation
Population-level use was confirmed by a systematic wastewater survey in Australia spanning 2022–2023, which detected 4-fluorophenibut alongside other novel psychoactive substances.[4] The compound remains unscheduled in most jurisdictions and continues to circulate through unregulated vendors.
The name "F-phenibut" reflects straightforward chemical shorthand: the "F" denotes the fluorine atom added to the phenibut scaffold at the para position of the phenyl ring. Vendor nomenclature varies — "4F-phenibut," "fluorophenibut," and "fluorobut" all refer to the same compound. Unlike phenibut, which carries decades of Soviet clinical heritage and a reputation as a nootropic, F-phenibut has no independent cultural identity beyond the research chemical community, where it is regarded as a more potent but less forgiving version of its parent compound.
Chemistry[edit]
Structural class
F-Phenibut (4-amino-3-(4-fluorophenyl)butanoic acid) is a fluorinated structural analog of phenibut belonging to the β-aryl-substituted GABA family. The defining modification is a fluorine atom at the para position of the phenyl ring, placing F-phenibut between unsubstituted phenibut and the para-chloro-substituted baclofen in a halogen series.[7][8]
The electron-withdrawing fluorine strengthens GABA-B binding, consistent with the approximately 58-fold potency increase over phenibut.[1] The related compound tolibut carries a methyl group at the same position — an alkyl rather than halogen substituent.
Molecular properties
| Molecular formula | C₁₀H₁₂FNO₂ |
| Molecular weight | 197.21 g/mol |
The IUPAC name listed on some reference sites ("5-amino-4-(4-fluorophenyl)-1-hydroxypentan-2-one") is incorrect; the verified name is 4-amino-3-(4-fluorophenyl)butanoic acid.[9][3]
Stereochemistry
F-phenibut has a chirality center at C-3 and is commercially available as a racemic mixture. No stereochemical studies of its individual enantiomers have been published.
By analogy with phenibut, where the R-enantiomer carries essentially all GABA-B agonist activity,[10] and with baclofen, where R(+)-baclofen is the pharmacologically dominant form,[11] the R-enantiomer is expected to account for the majority of F-phenibut's activity. The commercial racemate therefore contains approximately 50% inactive material by mass. inferred
Physical properties
F-phenibut is commercially supplied as the hydrochloride salt, a white crystalline powder.[3] No published data exist on melting point, aqueous solubility, or stability. The fluorine substitution is expected to modestly increase metabolic stability and CNS penetration relative to phenibut, though these properties have not been measured.
Pharmacology[edit]
Mechanism of action
F-phenibut activates the GABA-B, a G-protein coupled receptor that forms obligate two-part complexes in the brain.[12][2] When F-phenibut binds, the receptor triggers a signaling cascade that closes calcium channels, opens potassium channels, and reduces cellular excitability — the net result is widespread inhibition of neural activity.[2]
In laboratory testing on mouse brain cells, F-phenibut proved approximately 58 times more potent than phenibut and 4 times less potent than baclofen at activating GABA-B receptors.[1] It suppressed nerve cell firing consistent with its profile as a CNS depressant.[1]
A secondary mechanism — binding to the α₂-δ subunit of voltage-dependent calcium channels — is established for phenibut but has not been directly tested for F-phenibut.[13] This mechanism is shared by gabapentin and pregabalin and may contribute to anxiolysis and pain suppression independently of GABA-B activation. Whether F-phenibut retains this property is unknown. inferred
GABA-A activity is negligible — F-phenibut shows no meaningful binding at concentrations relevant to its GABA-B effects.[3]
Pharmacokinetics
No human pharmacokinetic studies exist for F-phenibut. All parameters are inferred from phenibut class data.
Phenibut reaches peak brain concentrations within approximately 15 minutes of administration in animal models, with an half-life of roughly 5–6 hours.[14][15] It undergoes virtually no liver metabolism and is excreted largely unchanged in urine.[14]
The fluorine substitution may modestly increase metabolic stability and brain penetration, but no direct measurements exist. Given the minimal liver metabolism of the phenibut class, drug interactions through metabolic competition are not expected but remain unstudied.
Neuroscience[edit]
Where it acts
F-phenibut’s effects arise from GABA-B activation across brain regions with high receptor density — the cortex, thalamus, cerebellum, hippocampus, and amygdala.[12] At presynaptic terminals, activation reduces the release of excitatory neurotransmitters including glutamate; at postsynaptic sites, it hyperpolarizes neurons directly.[2]
In cerebellar cells specifically, F-phenibut activates a potassium current distinct from the one seen in other brain regions.[1] This may explain why motor coordination impairment appears early — before heavy sedation — consistent with user reports of stumbling at doses that leave cognition relatively intact.
Why it matters
The anxiolytic and sedative effects are best understood through thalamocortical gating — GABA-B receptors in the thalamus regulate how much sensory information reaches the cortex, and enhanced inhibition at this level dampens arousal, producing calm at low occupancy and sedation at high occupancy.[2]
The dependence paradox is striking: baclofen reduces the rewarding properties of alcohol and stimulants,[11] yet phenibut-class compounds themselves carry high abuse liability. This suggests that negative reinforcement — escape from rebound effect anxiety between doses — drives compulsive use more powerfully than pleasure itself.
Key unknowns include whether F-phenibut binds the α₂-δ calcium channel subunit (which would make it a dual-mechanism compound like gabapentin rather than a pure GABA-B agonist), what the enantiomeric potency ratio is (a resolved R-form could be twice as potent as the racemic mixture), and whether the unusual potassium current seen in cerebellar cells[1] extends to other brain regions.
Subjective Effects[edit]
The F-phenibut experience is characterized by a dissolving of social anxiety and physical tension into a state of warm, heavy calm — similar in quality to phenibut but arriving at a fraction of the dose. At lower doses, users describe clear-headed confidence and mood elevation; at moderate doses, this gives way to thick sedation and motor impairment with little warning. Set and setting strongly modulate the response — a stimulating social environment may sustain prosocial effects at doses that would produce deep sedation in a quiet room, and users with baseline anxiety report a qualitatively different experience (primarily relief) compared to non-anxious users (primarily euphoria).
Within the β-aryl-GABA class, F-phenibut is most frequently compared to Phenibut but at dramatically compressed doses — where phenibut users report common effects at 750–1500 mg, F-phenibut achieves comparable activation at 150–400 mg.[9] The steeper dose-response curve leaves less margin between the desired anxiolytic effect and overwhelming sedation, making precise dosing more critical than with the parent compound.
Effect Grid
At common doses (150–400 mg oral), the full anxiolytic and prosocial profile emerges. Anxiety suppression is pronounced — social inhibitions diminish substantially, and users report cognitive euphoria, motivation enhancement, and a sense of emotional openness. Disinhibition is marked, and empathy enhancement contributes to sociability. Music enhancement is frequently noted. A compulsive redosing urge may appear at the upper end of this range.
Physically, muscle relaxation is significant. Tactile enhancement and a diffuse bodily warmth are commonly described. Motor control impairment begins to manifest — gait may become slightly unsteady. Mild sedation competes with stimulation at the lower end of this range.
At the upper end, thought deceleration and sedation increase. Sleep becomes difficult to resist if the user is in a resting environment. The experience lasts 6–8 hours from onset.
Modeled from the effect's dose-dependence curve — not per-substance measurements.
Spatial disorientation · Vestibular distortion · Visual acuity suppression · Visual haze / noise
Body high · Physical fatigue · Tactile euphoria · Dizziness · Dry mouth · Pain suppression · Sexual arousal enhancement · Stimulation · Nausea · Dehydration sensation · Headache · Insomnia · Spontaneous body sensations · Yawning · Excessive sweating · Muscle tension · Increased urination · Temperature dysregulation · Constipation · Laughter fits · Nystagmus (eye wobbles) · Restlessness · Tingling / electric sensations · Vibrations / buzzing · Wakefulness · Appetite enhancement · Abnormal heartbeat · Involuntary body movements · Vomiting · Bodily dissociation
Dizziness · Dry mouth · Pain suppression · Sexual arousal enhancement · Stimulation · Nausea · Dehydration sensation · Headache · Insomnia · Spontaneous body sensations · Yawning · Excessive sweating · Muscle tension · Increased urination · Constipation · Laughter fits · Nystagmus (eye wobbles) · Restlessness · Tingling / electric sensations · Vibrations / buzzing · Wakefulness · Appetite enhancement · Abnormal heartbeat · Involuntary body movements · Vomiting · Bodily dissociation
Analysis suppression · Thought deceleration · Information processing suppression · Openness enhancement · Motivation enhancement · Focus enhancement · Aesthetic enhancement · Delusions of sobriety · Immersion enhancement · Memory suppression · Motivation suppression · Suggestibility enhancement · Creativity enhancement · Humor enhancement · Language enhancement · Cognitive dysphoria · Confusion · Introspection enhancement · Memory fragmentation · Thought disorganization · Amnesia · Delirium
Openness enhancement · Motivation enhancement · Focus enhancement · Aesthetic enhancement · Immersion enhancement · Memory suppression · Motivation suppression · Suggestibility enhancement · Creativity enhancement · Humor enhancement · Language enhancement · Cognitive dysphoria · Confusion · Introspection enhancement · Memory fragmentation · Thought disorganization · Amnesia · Delirium
Anxiety · Emotional enhancement · Irritability · Dysphoria · Emotional suppression · Emotional lability
Emotional suppression · Emotional lability
Social connection · Trust enhancement · Compulsive behavior reduction · Ego inflation · Emotional susceptibility · Flow state · Communication suppression · Sociability suppression · Boundary dissolution
Emotional susceptibility · Flow state · Communication suppression · Sociability suppression · Boundary dissolution
Timeline
First alerts emerge as a subtle reduction in background anxiety and a faint sense of physical relaxation. Effects may be easily overlooked initially.
Anxiolysis becomes clearly perceptible. Muscle relaxation deepens, social inhibition loosens, and a gentle mood lift builds steadily.
Maximum anxiolysis, cognitive euphoria, and prosocial effects. At higher doses, sedation and motor impairment dominate. Compulsive redosing urge may be strongest during this phase.
Effects sustain at reduced intensity. Anxiolysis persists but euphoria fades. Sedation may increase as stimulating qualities wane.
Gradual return toward baseline. Residual muscle relaxation and mild sedation persist. Some rebound anxiety may emerge in regular users.
Residual relaxation and mild mood changes may persist for several hours after primary effects resolve. Some users report next-day sedation or cognitive fog at higher doses.
⚠ Do not drive or operate heavy machinery until you have slept a full night and feel fully alert, as residual sedation and motor impairment may persist beyond the subjective experience.
Safety[edit]
- Acute toxicity
- Moderate
- Chronic toxicity
- Moderate
- Withdrawal
- Severe
- Tolerance buildup
- rapid
- Tolerance reset
- 10.5 days
- Physical
- Moderate
- Psychological
- Moderate
- Compulsive redosing
- Moderate
- Dose escalation
- High
- Cross-tol.
- phenibut · baclofen · GHB
Toxicity
Acute toxicity
No lethal dose data exist for F-phenibut in any species, and no formal toxicity study has been conducted. safety citation needed
The primary acute danger is F-phenibut's approximately 58-fold higher potency relative to phenibut.[1] Users transitioning from phenibut without appropriate dose reduction face substantial overdose risk. Severe intoxication is expected to resemble phenibut overdose: progressive CNS depression from sedation through coma, respiratory depression requiring mechanical ventilation, and injuries from falls.[14]
More than 80 cases of coma and death have been associated with phenibut use and withdrawal in the United States, most in polydrug contexts.[16] Partial loss of brainstem reflexes[17] and dissociative features[18] have been documented during phenibut intoxication.
No specific antidote exists for GABA-B receptor agonist overdose. Treatment is supportive: airway protection, breathing support, and monitoring of heart rate and blood pressure.[14][19]
Detection
Standard hospital drug screens do not detect phenibut or F-phenibut. Identification requires specialized mass spectrometry methods not routinely available in emergency departments.[19][20] Clinicians managing suspected overdose must rely on patient history and symptoms alone.
Psychological risks
No controlled data on psychiatric adverse effects of F-phenibut exist. By class inference from phenibut, acute psychological risks at high doses include rebound effect anxiety, memory suppression, amnesia, and dissociative symptoms.[18] Psychosis has been documented during both intoxication and withdrawal.[5]
The primary psychiatric risk is self-reinforcing dependence. F-phenibut's anxiolytic effect creates a cycle in which inter-dose rebound anxiety drives escalating use — the compound becomes self-medication for a withdrawal-generated symptom.
Tolerance and dependence
Tolerance develops within days of continuous use, consistent with phenibut class behavior.[5][21] No controlled study has quantified the rate. A community-reported tolerance reset time of approximately 10.5 days circulates online but has no peer-reviewed basis. citation needed
Physical dependence develops with regular use. Withdrawal symptoms can begin within 3–4 hours of the last dose[21] and include severe agitation, unstable heart rate and blood pressure, psychosis, and seizures.[5][6] F-phenibut's higher potency relative to phenibut[1] suggests potentially faster dependence at lower absolute doses, though this has not been formally studied.
Medical withdrawal management typically uses baclofen substitution and taper at approximately 10 mg baclofen per 1 g phenibut equivalence.[21] Alternatives include benzodiazepines and phenobarbital, with phenobarbital preferred when seizure risk is elevated.[22]
Drug interactions
Opioids pose the highest risk. Additive breathing suppression from concurrent GABA-B and µ-opioid receptor activation can be fatal.[23]
Benzodiazepines produce additive CNS depression through convergent GABAergic mechanisms. Polydependence on phenibut and benzodiazepines has been documented, requiring coordinated withdrawal management.[24]
Alcohol creates a complex interaction: GABA-B agonism may reduce alcohol's rewarding properties,[11] but concurrent use produces additive depression with increased risk of breathing failure and aspiration.
Other GABA-B agonists (Baclofen, GHB, GBL) produce additive GABA-B activation with risk of severe CNS depression.[25]
Gabapentinoids (gabapentin, pregabalin) share the calcium channel binding mechanism inferred for F-phenibut, creating potential for additive effects.
No interaction studies exist specifically for F-phenibut; all severity assessments are class-level inferences. safety citation needed Given minimal liver metabolism of the phenibut class,[14] metabolic drug interactions are not expected but remain unstudied.
Contraindications
Breathing disorders — GABA-B agonism produces dose-dependent breathing suppression. Pre-existing conditions such as COPD, sleep apnea, or obesity hypoventilation increase the risk of respiratory failure.
Concurrent depressant use — Co-administration with opioids, benzodiazepines, alcohol, barbiturates, or other GABA-B agonists substantially increases overdose risk.
Seizure disorders — Acute GABA-B agonism has anticonvulsant properties, but abrupt discontinuation after regular use may trigger seizures.[5]
Pregnancy and breastfeeding — No reproductive safety data exist. GABA-B agonists cross the placenta; F-phenibut should be considered contraindicated pending data. safety citation needed
Kidney impairment — Phenibut is excreted largely unchanged in urine.[14] Impaired kidney function could prolong F-phenibut exposure and increase toxicity risk.
Harm Reduction[edit]
Forms & Identification
F-phenibut is almost exclusively sold as a white crystalline powder (hydrochloride salt) through online research chemical and nootropic vendors. It is visually indistinguishable from phenibut, and because F-phenibut is roughly 58 times more potent, product mislabeling or confusion between the two compounds represents a serious overdose risk.
Milligram-precision scales are essential — kitchen scoops and volumetric estimation are inadequate for a substance where a 50 mg error meaningfully shifts the response. If purchasing from an unfamiliar source, third-party laboratory analysis is the only reliable way to confirm identity and purity.
Guidelines
- Use a milligram-precision scale for every dose — never estimate.
- Start at the lowest reported threshold and wait the full onset period before considering redosing.
- Do not use on consecutive days. Tolerance develops within days, and physical dependence follows quickly.
- Keep a written log of doses and dates to detect escalation patterns early.
- Never combine with opioids, alcohol, benzodiazepines, GHB/GBL, or other GABA-B receptor agonists.
- Avoid driving or operating machinery — motor impairment can appear before subjective sedation.
- If transitioning from phenibut, do not assume a simple potency conversion. Start well below the calculated equivalent and titrate.
Set & Setting
F-phenibut's steep dose-response curve and narrow window between anxiolysis and heavy sedation make environment unusually important. A stimulating social setting can sustain prosocial effects at doses that would produce deep sedation in a quiet room. Fall risk increases significantly at moderate-to-high doses — choose environments without stairs, sharp edges, or water hazards.
Psychological set matters: users with untreated anxiety disorders are at elevated risk of developing rapid dependence through negative reinforcement cycling.
Aftercare & integration. Post-experience recovery from F-phenibut is primarily physical. Prioritize sleep, hydration, and light nutrition. Rebound anxiety is common in the 12–24 hours following a dose and does not require redosing — it resolves on its own.
If F-phenibut was used to manage social anxiety or stress, identify and address the underlying driver. The compound's rapid tolerance development means it cannot serve as a sustainable anxiety management tool. Consider whether the experience revealed unmet needs that could be addressed through therapy, lifestyle changes, or other support.
Recovery & Support
F-phenibut dependence can develop within days to weeks of regular use, and unsupervised withdrawal carries serious medical risks including seizures. Do not attempt to stop abruptly after a period of daily use.
Medical tapering — typically using baclofen as a substitute — is the safest approach and should be supervised by a healthcare provider. If a provider is unfamiliar with phenibut-class withdrawal, the baclofen equivalence protocol (approximately 10 mg baclofen per 1 g phenibut equivalent) documented in case reports can serve as a starting reference.[21]
Support resources include SAMHSA's National Helpline (1-800-662-4357), online communities focused on GABAergic substance recovery, and addiction medicine specialists.
Recovery resources, crisis lines, and support pathways → View all
Emergency Response
Recognizing an emergency: Unrousable sedation, slow or shallow breathing, blue-tinged lips or fingertips, vomiting while unconscious, or seizures. These distinguish dangerous overdose from uncomfortable but tolerable sedation.
Immediate steps:
- Call emergency services immediately if the person cannot be woken or is breathing fewer than 10 times per minute.
- Place the person in the recovery position (on their side) to prevent choking on vomit.
- Do not leave them alone.
- Do not induce vomiting.
- Monitor breathing continuously until help arrives.
What to tell the dispatcher: Name the substance (F-phenibut / fluorophenibut), estimated amount and time of ingestion, current symptoms, and whether other substances were involved. Good Samaritan laws in many jurisdictions protect callers from prosecution.
Note: No reversal agent exists for GABA-B agonist overdose. Treatment is supportive — the priority is protecting the airway and maintaining breathing.
Legal Status[edit]
F-phenibut occupies an ambiguous legal position in most jurisdictions. It is not internationally controlled — the WHO Expert Committee on Drug Dependence has had phenibut under surveillance since 2017 and conducted a pre-review in 2021, but no international scheduling recommendation has been issued.[26] F-phenibut's legal status in any given country depends on whether it is named explicitly or captured under broader structural analog or novel psychoactive substance frameworks.
| Jurisdiction | Classification | Key provisions |
|---|---|---|
| United States | Not scheduled | Not a controlled substance under the CSA; FDA has issued warnings against phenibut as a dietary ingredient but has taken no specific action against F-phenibut[16] |
| United Kingdom | Controlled (Psychoactive Substances Act 2016) | Sale, supply, and import prohibited; personal possession not criminalized |
| Germany | Illegal (NpSG) | Prohibited under the New Psychoactive Substances Act[14] |
| France | Prohibited | Banned under NPS legislation[14] |
| Italy | Controlled | Prohibited under NPS legislation[14] |
| Hungary | Specifically prohibited | Named compound ban; F-phenibut explicitly listed[3] |
| Lithuania | Specifically prohibited | Named compound ban; F-phenibut explicitly listed[3] |
| Australia | Ambiguous | Phenibut is Schedule 9 (prohibited, 2018); whether F-phenibut is captured under this scheduling is unclear. Detected in Australian wastewater, confirming circulation[4] |
| Canada | Uncontrolled | Neither phenibut nor F-phenibut is a controlled substance |
| Russia | Phenibut: prescription drug | Phenibut is a registered medication; F-phenibut has no approved medical use[7] |
Analog act considerations
In the United States, the Federal Analogue Act theoretically applies to substances "substantially similar" to Schedule I or II controlled substances when intended for human consumption. However, phenibut itself is not scheduled under the Controlled Substances Act, which complicates analog act application to F-phenibut — the act requires the reference compound to be Schedule I or II.
references
- [1] ^abcdefghijIrie T, Yamazaki D, Kikura-Hanajiri R (2020) F-phenibut (beta-(4-Fluorophenyl)-GABA), a potent GABA-B receptor agonist, activates an outward-rectifying K+ current and suppresses action potentials in mouse cerebellar Purkinje cells — European Journal of Pharmacology doi:10.1016/j.ejphar.2020.173437
- [2]
- [3]
- [4] ^abcJaunay EL, Bade R, Paxton KR, Nadarajan D, Barry DC, Zhai Y, Tscharke BJ, O'Brien JW, Mueller J, White JM, Simpson BS, Gerber C (2024) Monitoring the use of novel psychoactive substances in Australia by wastewater-based epidemiology — The Science of the Total Environment doi:10.1016/j.scitotenv.2024.170473
- [5]
- [6] ^abHardman MI, Sprung J, Weingarten TN (2019) Acute phenibut withdrawal: A comprehensive literature review and illustrative case report — Bosnian Journal of Basic Medical Sciences doi:10.17305/bjbms.2018.4008