Gabapentin
Gabapentin
Neurontin (IR), Gralise (ER), Horizant (gabapentin enacarbil ER)
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Summary
Common uses
Partial-onset seizures (adjunctive, FDA)0, Postherpetic neuralgia (FDA)0, Restless legs syndrome (Horizant, FDA)0, Neuropathic pain (off-label, broad)0, Fibromyalgia (off-label)0, Anxiety disorders (off-label, evidence mixed)0, Alcohol use disorder (off-label)0, Menopausal hot flashes (off-label)0
Pharmacy
Starting dose
300 mg PO at bedtime night 1, 300 mg BID day 2, 300 mg TID day 3; titrate to clinical effect, commonly 1800-3600 mg/day divided TID
Preparations
Capsules 100, 300, 400 mg; tablets 600, 800 mg; oral solution 250 mg/5 mL; Gralise ER tablets 300, 600 mg (once-daily); Horizant ER tablets 300, 600 mg (gabapentin enacarbil, an inactive parent compound metabolized to gabapentin in vivo)
US FDA Max
3600 mg/day; off-label doses higher are common but bioavailability saturates well below this
Pharmacology
Routes
Oral
Onset
1-2 weeks for neuropathic pain and anxiolytic effect; anticonvulsant effect at therapeutic plasma level
Duration
TID dosing for IR; once-daily for ER formulations
Half-life
5-7 hours[1]
Bioavailability
Saturable via the LAT-1 amino-acid transporter, producing nonlinear pharmacokinetics: ~60% at 300 mg single dose, falling to ~35% at 1200 mg single dose[1]
Pregnancy
Limited human data; some signal for cardiac malformations and developmental delay but confounded by maternal disease and polytherapy.[citation needed]
Legal status
Rx-only federally in US. State-level scheduling: Schedule V in Kentucky, Tennessee, Virginia, North Dakota, Michigan, Alabama, Utah, Wyoming, North Carolina, Mississippi, Louisiana, Connecticut, Ohio (and growing), reflecting documented misuse and overdose-potentiation in combination with opioids[1]
Purported mechanism
Despite the name, gabapentin does not bind GABA receptors or affect GABA reuptake. It binds the α2δ-1 auxiliary subunit of voltage-gated calcium channels, reducing presynaptic Ca²⁺ influx and consequent release of excitatory neurotransmitters (glutamate, substance P, norepinephrine, calcitonin gene-related peptide). The pain and anticonvulsant effects share this presynaptic-excitability dampening mechanism.0 Saturable LAT-1 absorption is the dominant clinical-pharmacokinetic quirk and explains why dose increases above ~1800 mg/day yield diminishing plasma exposure. Horizant addresses this through the enacarbil parent compound, which is absorbed by non-saturable transporters and then metabolized to gabapentin in vivo[1].
References
- ↑ 1.0 1.1 1.2 1.3 FDA Prescribing Information, Neurontin (gabapentin), Pfizer, current revision. https://www.accessdata.fda.gov/drugsatfda_docs/label/2017/020235s064_020882s047_021129s046lbl.pdf