F-Phenibut Facts
Depressant;
Description
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.
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.
F-phenibut produces rapid physical dependence, and withdrawal can include seizures, psychosis, and dangerous instability in heart rate and blood pressure.[2][3] Its 58-fold potency over phenibut means milligram-level miscalculations can shift a dose from anxiolytic to dangerous.[1]
Dose and durationby route · individual sensitivity varies
Starts in 20 – 60 minLasts 6 – 8 hoursAfter-effects 2 – 6 hours
Body and dependence
- Acute toxicity
- Moderate
- Chronic toxicity
- Moderate
- Physical dependence
- Moderate
- Psychological dependence
- Moderate
- Withdrawal
- Severe · medical supervision
- Compulsive redosing
- Moderate
Tolerance
- Builds
- Rapid
- Fully resets after
- 10.5 days
- Carries over to
- phenibut;
baclofen; GHB; GBL; alcohol; benzodiazepines
Effectslikely at a common dose
- Perception
- none likely · 2 possible
- Body
- Muscle relaxation;
Sedation; +16 possible, including Motor control impairment, Dizziness, Nausea - Thinking
- none likely · 20 possible, including Cognitive impairment, Compulsive redosing urge, Analysis suppression
- Feeling
- Anxiety suppression;
+8 possible, including Anxiety, Depression, Dysphoria - Self
- none likely · 10 possible, including Craving, Ego inflation
- Awareness
- none likely · 1 possible
Who shouldn't take it
Combinations60 recorded
Seek help immediately if
- Extreme drowsiness — can't stay awake or be roused
- Confusion, slurred speech, severe loss of coordination
- Slow, shallow, or irregular breathing
- Unconsciousness / unresponsive; limp, floppy body
- Blue lips or fingertips
- Vomiting while sedated (choking / aspiration risk)
- Cold, clammy skin; weak pulse
What to do
- Try to wake them — shout, firm sternal rub
- If unresponsive or breathing is impaired, call emergency services
- Place them in the recovery position — critical, they can choke on vomit
- Monitor breathing continuously; be ready to give rescue breaths / CPR
- Never leave them alone to "sleep it off"
- Do not give other drugs, stimulants, or more depressants
Most depressant overdoses resolve with airway protection, breathing support, and monitoring. The danger is respiratory depression and choking on vomit — sharply worse when combined with opioids or alcohol. GHB/GBL overdoses often involve sudden deep unconsciousness and may self-resolve, but airway protection is essential.
988 Suicide & Crisis LifelineFireside Project: 62-FIRESIDE
References
- [1]^abIrie 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]^Weleff J, Kovacevich A, Burson J, Nero N, Anand A (2023) Clinical Presentations and Treatment of Phenibut Toxicity and Withdrawal: A Systematic Literature Review — Journal of Addiction Medicine doi:10.1097/adm.0000000000001141
- [3]^Hardman 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