Baclofen

Selective GABA-B agonist with A-tier evidence for MS / spinal-cord spasticity (FDA-approved 1977), B-tier evidence for alcohol use disorder (the "French exception" — approved for AUD in France 2018 after Olivier Ameisen's 2005 self-cure case + ALPADIR/Bacloville trials), and B-tier-at-best evidence for anxiety. For Dylan: no spasticity, no alcohol use, anxiety already covered by V4 — there is no use case. The kicker is the discontinuation profile: abrupt stop after even moderate chronic use can produce delirium, hyperactive psychosis, autonomic instability, status epilepticus, and (for intrathecal forms) NMS-like multi-organ failure within 1-4 days. SKIP-FOR-NOW LOW.

Baclofen is β-(4-chlorophenyl)-γ-aminobutyric acid — chemically GABA with a chlorinated phenyl group attached. It is the structural twin of phenibut (which is the same molecule with H instead of Cl), and the two compounds share the GABA-B receptor agonist mechanism but differ enormously in potency and breadth of activity.

Primary action — GABA-B receptor agonism (selective):

The GABA-B receptor is a metabotropic G-protein coupled receptor (Gi/o family), distinct from the ionotropic GABA-A receptor that benzodiazepines, barbiturates, and Z-drugs target. It exists as an obligate heterodimer of GABA-B1 and GABA-B2 subunits. Baclofen binds the orthosteric GABA site on the GABA-B1 subunit with Ki ~6 µM — making it the most potent of the GABA analogues at this receptor (vs phenibut at ~92 µM, ~30-68× weaker; gabapentin and pregabalin do not meaningfully bind GABA-B at all).

GABA-B activation produces two complementary inhibitory effects:

1. Presynaptic inhibition. GABA-B receptors on presynaptic terminals couple to Gi proteins, which inhibit adenylyl cyclase and close N-type and P/Q-type voltage-gated calcium channels. Less calcium influx → less neurotransmitter vesicle fusion → reduced release of excitatory neurotransmitters (glutamate, aspartate, substance P). This is the dominant mechanism for baclofen's antispastic action at the spinal cord level — it reduces the glutamatergic drive onto motor neurons, suppressing mono- and polysynaptic spinal reflexes.

2. Postsynaptic inhibition. GABA-B receptors on postsynaptic membranes activate G-protein coupled inwardly-rectifying potassium channels (GIRKs), driving K⁺ efflux and hyperpolarizing the neuron. Hyperpolarized neurons are harder to fire — so baclofen indirectly raises the threshold for excitatory drive throughout the CNS.

Neuroanatomic distribution. GABA-B receptors are densest in cortex, thalamus, cerebellum, and spinal cord. Baclofen's antispastic indication is driven by spinal action (reduced motor neuron excitability). Its centrally-mediated effects — sedation, anxiolysis, mild euphoria, dissociation at high doses, and the alcohol-craving suppression — are driven by supraspinal action, particularly in the ventral tegmental area (VTA) where GABA-B activation depresses dopamine neuron firing, and in the amygdala / serotonergic raphe where GABA-B activation acutely inhibits serotonin release.

For alcohol use disorder specifically: the proposed mechanism is GABA-B-mediated suppression of mesolimbic dopamine reward signaling from alcohol cues, plus reduction in stress-driven craving via amygdala GABA-B effects. The animal data is fairly clean; the human translation is contested.

Pharmacokinetics:

• Oral bioavailability ~70-85% (rapid GI absorption — does not cross blood-brain barrier as efficiently as the more lipophilic phenibut; this is part of why baclofen's ceiling effect is sharper and high-dose therapy is needed for AUD).
• Peak plasma concentration: 1-2 hours.
• Plasma half-life: 3-4 hours — short. This drives the TID/QID dosing in spasticity (5-20 mg three to four times daily) and the load-and-titrate complexity in AUD (total daily doses 30-300 mg, divided).
• ~70-80% renal excretion of unchanged drug. Renal impairment substantially extends half-life.
• Crosses BBB but with relatively low CNS penetration vs phenibut (the chloro group reduces lipophilicity vs phenibut's plain phenyl). This is why intrathecal baclofen exists for severe spasticity — direct spinal delivery bypasses the BBB ceiling.

Compared to its siblings:

• Phenibut: structural twin (H in place of Cl), but ~30-68× weaker at GABA-B and also active at α2δ voltage-dependent calcium channels (the gabapentinoid site). Phenibut is therefore a dual GABA-B + α2δ ligand with much greater BBB penetration — characteristic euphoria, anxiolysis, and severe dependence/withdrawal on chronic use.
• Gabapentin: does not bind GABA-B. Acts at α2δ-1 calcium channel subunit only.
• Pregabalin: same α2δ mechanism as gabapentin, higher potency and bioavailability. Not a GABA-B drug.

So among the four "GABA-named" compounds, baclofen is the only pure GABA-B agonist; phenibut is dual-mechanism; gabapentin/pregabalin are pure α2δ ligands and not GABA-B at all.