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, roughly 58 times more potent by weight.[1] It activates GABA-B receptors — the brain's main inhibitory system — producing sedation, muscle relaxation, and calm.
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Synthesized by Ciba-Geigy under the developmental code CGP-11130, F-phenibut was never brought to clinical trials or approved for medical use in any jurisdiction.[2] It appeared on online nootropic and research chemical markets in the 2010s, marketed as a more potent alternative to phenibut. It remains unscheduled in most countries, circulates through unregulated vendors, and has been detected in Australian wastewater surveys confirming population-level use.[3]
Subjective effects include anxiety relief, sedation, mood elevation, muscle relaxation, and sociability enhancement. The experience is a dissolution of anxiety into warm, heavy calm — similar to phenibut but arriving at a fraction of the dose. Motor impairment, nausea, and excessive drowsiness are common, and the shift from comfortable relaxation to incapacitating sedation can happen with little warning.
F-phenibut produces rapid physical dependence, and withdrawal can include seizures, psychosis, and dangerous instability in heart rate and blood pressure.[4][5] Its 58-fold potency over phenibut means milligram-level miscalculations can shift a dose from anxiolytic to dangerous.[1] No controlled human studies exist — the evidence rests on a single laboratory study, class inference from phenibut and baclofen, and user reports.
History[edit]
Discovery
F-Phenibut was synthesized by Ciba-Geigy (now Novartis) as a research compound — code name CGP-11130 — during systematic work on GABA analogs.[2] It sits between two better-known relatives: phenibut, used as an anxiolytic in the Soviet Union since the 1960s,[6] and baclofen, the only member of the series to win Western regulatory approval. It was never advanced to clinical trials.
Market appearance
F-phenibut surfaced on online nootropic and research chemical markets in the 2010s under names including "fluorophenibut" and "4F-phenibut." Its first scientific characterization came in 2020, when researchers used mouse brain cells to confirm its GABA-B potency — approximately 58 times greater than phenibut.[1]
Present circulation
Population-level use was confirmed by Australian wastewater surveys spanning 2022–2023, which detected F-phenibut alongside other novel psychoactive substances.[3] The compound remains unscheduled in most jurisdictions and continues to circulate through unregulated vendors.
The name "F-phenibut" is simple chemical shorthand: the "F" marks the fluorine atom added to the phenibut scaffold at the para position of the phenyl ring. Vendor names vary — "4F-phenibut," "fluorophenibut," and "fluorobut" all refer to the same compound. Unlike phenibut, which carries decades of Soviet clinical heritage, 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.
Chemistry[edit]
Structural class
F-Phenibut (4-amino-3-(4-fluorophenyl)butanoic acid) belongs to the β-aryl-substituted GABA family — the same class as phenibut and baclofen.[6][7]
A fluorine atom at the para position of the phenyl ring is the only structural difference from phenibut, but this modification strengthens GABA-B binding and accounts for the approximately 58-fold potency increase.[1] Its molecular formula is C₁₀H₁₂FNO₂ (molecular weight 197.21 g/mol).[2]
Pharmacology[edit]
Mechanism of action
F-phenibut activates GABA-B receptors — the brain's primary off-switches for neural activity.[8][9] When these receptors fire, calcium channels close and potassium channels open, reducing nerve cell activity across the brain. The net result is broad inhibition: anxiety fades, muscles relax, and at higher occupancy sedation takes over.
In laboratory testing, F-phenibut proved about 58 times more potent than phenibut and 4 times less potent than baclofen at activating GABA-B receptors.[1] A possible secondary mechanism — binding calcium channel proteins shared with gabapentin and pregabalin — is established for phenibut but has not been tested for F-phenibut.[10] GABA-A activity is negligible at relevant concentrations.[2]
Pharmacokinetics
No human pharmacokinetic studies exist. By inference from phenibut, F-phenibut likely reaches peak brain concentrations within about 15 minutes, with an half-life of roughly 5–6 hours.[11][12] The phenibut class undergoes minimal liver metabolism and is excreted largely unchanged in urine,[11] so metabolic drug interactions are unlikely but remain unstudied.
Neuroscience[edit]
Where it acts
F-phenibut's effects arise from GABA-B activation across the cortex, thalamus, cerebellum, hippocampus, and amygdala.[8] At nerve terminals it reduces the release of excitatory signals; at postsynaptic cells it slows firing directly.[9] In cerebellar cells specifically, F-phenibut triggers a distinct potassium current[1] — which may explain why coordination fails before heavy sedation sets in.
Why it matters
GABA-B receptors in the thalamus regulate how much sensory input reaches the cortex — enhanced inhibition at this level dampens arousal, producing calm at low doses and sedation at high doses.[9] Dependence is driven primarily by negative reinforcement: between doses, rebound effect anxiety pushes toward the substance. Notably, baclofen — F-phenibut's chloro-substituted relative — actually reduces alcohol cravings,[13] yet phenibut-class compounds themselves carry high abuse liability.
Key unknowns: whether F-phenibut binds the calcium channel proteins shared with gabapentin (which would make it dual-mechanism rather than a pure GABA-B agonist), what the potency difference is between its two mirror-image molecular forms, and whether the unusual potassium current seen in cerebellar cells[1] extends to other brain regions. No controlled human dose-response study exists, and the boundary between anxiolysis and dangerous CNS depression has never been formally characterized.
Subjective Effects[edit]
The F-phenibut experience is a gradual dissolution of anxiety and physical tension into warm, heavy calm. At lower doses, social ease and quiet mood elevation dominate. The dose-response curve is steep — moderate doses push quickly into thick sedation and motor impairment — leaving little room between the pleasant state and an overwhelming one. Set and setting shape the experience more than with many depressants.
Within the β-aryl-GABA class, F-phenibut is closest to phenibut in character — the same quality of anxiolytic relief, compressed into far smaller doses. Where phenibut users often describe a gradual, manageable arc, F-phenibut's steeper curve leaves less margin to find the desired effect without overshooting into incapacitating sedation.
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 Its 58-fold higher potency than phenibut[1] creates substantial overdose risk when users miscalculate dose conversions. Severe overdose is expected to resemble phenibut: progressive CNS depression from sedation through coma, respiratory depression requiring mechanical ventilation, and injuries from falls.[11] More than 80 cases of coma and death have been associated with phenibut use and withdrawal in the United States, most in polydrug contexts.[14] Partial loss of brainstem reflexes[15] and dissociative features[16] have been documented in phenibut intoxication. No antidote exists — treatment is airway protection and breathing support.[11][17]
Detection
Standard hospital drug screens do not detect F-phenibut. Identification requires specialized mass spectrometry not routinely available in emergency departments.[17][18] Clinicians treating suspected overdose must rely on patient history and symptoms alone.
Psychological risks
No controlled data on psychiatric effects of F-phenibut exist. By class inference, high doses can produce rebound effect anxiety, memory suppression, amnesia, and dissociative symptoms.[16] Psychosis has been documented during both intoxication and withdrawal.[4]
The primary psychiatric risk is self-reinforcing dependence. F-phenibut's anxiolytic effect generates rebound anxiety between doses, which drives escalating use — the compound becomes self-medication for a withdrawal-generated symptom.
Tolerance and dependence
Tolerance develops within days of continuous use.[4][19] A community-cited reset time of approximately 10.5 days has no peer-reviewed basis. citation needed
Physical dependence develops with regular use. Withdrawal can begin within 3–4 hours of the last dose[19] and include severe agitation, unstable heart rate and blood pressure, psychosis, and seizures.[4][5] F-phenibut's higher potency[1] suggests dependence may develop faster at lower absolute doses, though this is unstudied. Medical withdrawal typically uses baclofen substitution and taper (approximately 10 mg baclofen per 1 g phenibut equivalent),[19] with phenobarbital preferred when seizure risk is elevated.[20]
Drug interactions
Opioids pose the highest risk. Concurrent use can fatally suppress breathing through additive effects on the brainstem's respiratory control.[21]
Benzodiazepines produce additive CNS depression through overlapping GABAergic mechanisms. Combined dependence requires coordinated withdrawal management.[22]
Alcohol produces additive CNS depression with increased risk of breathing failure and aspiration.[13]
Other GABA-B agonists (Baclofen, GHB, GBL) produce additive GABA-B activation with risk of severe CNS depression.[23]
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 Metabolic interactions are unlikely given the phenibut class's minimal liver metabolism,[11] but remain unstudied.
Contraindications
Breathing disorders — GABA-B agonism suppresses breathing in a dose-dependent way. 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 is anticonvulsant, but abrupt discontinuation after regular use can trigger seizures.[4]
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.[11] Impaired kidney function could prolong F-phenibut exposure and increase toxicity risk.
Harm Reduction[edit]
Forms & Identification
F-phenibut is sold almost exclusively as a white crystalline powder (hydrochloride salt) through online research chemical and nootropic vendors. It is visually identical to phenibut powder. Because F-phenibut is roughly 58 times more potent, confusing the two is a serious overdose risk.
Milligram-precision scales are essential — kitchen scoops and volumetric estimates are inadequate for a substance where a 50 mg error meaningfully shifts the response. 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 cause deep sedation in a quiet room.
Fall risk increases significantly at moderate-to-high doses — choose environments without stairs, sharp edges, or water hazards. Those with untreated anxiety disorders face 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, the underlying driver warrants attention. 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. The baclofen equivalence protocol (approximately 10 mg baclofen per 1 g phenibut equivalent) documented in case reports can serve as a starting reference.[19]
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 — not internationally controlled, but captured by national NPS legislation in several countries.
| 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[14] |
| 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[11] |
| France | Prohibited | Banned under NPS legislation[11] |
| Italy | Controlled | Prohibited under NPS legislation[11] |
| Hungary | Specifically prohibited | Named compound ban; F-phenibut explicitly listed[2] |
| Lithuania | Specifically prohibited | Named compound ban; F-phenibut explicitly listed[2] |
| Australia | Ambiguous | Phenibut is Schedule 9 (prohibited, 2018); whether F-phenibut is captured under this scheduling is unclear. Detected in Australian wastewater, confirming circulation[3] |
| 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[6] |
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.
Legal status changes frequently and varies by jurisdiction. Nothing here constitutes legal advice. Verify current regulations in the relevant jurisdiction before making any decisions.
references
- [1] ^abcdefghiIrie 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] ^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
- [4]
- [5] ^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
- [6]