GABAPENTIN
Clinical safety rating: safe
Animal studies have demonstrated safety
Gabapentin is a structural analog of GABA but does not bind to GABA receptors. It binds to the α2δ subunit of voltage-gated calcium channels, reducing calcium influx and decreasing the release of excitatory neurotransmitters.
| Metabolism | Gabapentin is not significantly metabolized in humans; it is eliminated renally as unchanged drug. No involvement of hepatic cytochrome P450 enzymes. |
| Excretion | Renal: 76-81% unchanged in urine; biliary/fecal: <5% as metabolites; remainder (10-20%) as minor metabolites via urine. |
| Half-life | 5-7 hours (normal renal function); prolonged to 50-140 hours in end-stage renal disease; half-life independent of dose due to linear kinetics. |
| Protein binding | <3% (negligible); primarily to albumin; free fraction >97%. |
| Volume of Distribution | 0.65-1.04 L/kg (mean ~0.8 L/kg); distributes into extravascular tissues, including brain; not extensive binding. |
| Bioavailability | Oral: 60-75% (dose-dependent; decreases with increasing dose due to saturable transport); no parenteral formulation available. |
| Onset of Action | Oral: 1-2 hours (peak plasma concentration); immediate-release: seizure reduction within 1-2 hours; extended-release: ~2-3 hours. |
| Duration of Action | 8-12 hours (immediate-release, three-times-daily dosing); extended-release formulations provide 12-24 hours dosing interval. |
| Molecular Weight | 171.24 |
Initial dose: 300 mg orally once daily on day 1, 300 mg twice daily on day 2, then 300 mg three times daily on day 3; titrate as needed up to 1800-3600 mg/day in three divided doses. Maximum single dose: 1200 mg. Dose adjustments for renal impairment should be made based on creatinine clearance.
| Dosage form | TABLET |
| Renal impairment | For CrCl >= 60 mL/min: 300-1200 mg three times daily. For CrCl 30-59 mL/min: 400-1400 mg daily in two divided doses. For CrCl 15-29 mL/min: 200-700 mg once daily. For CrCl <15 mL/min: 100-300 mg once daily. Hemodialysis: Supplemental dose of 125-300 mg after each 4-hour dialysis session. |
| Liver impairment | No specific dose adjustment recommended for hepatic impairment. Use with caution in severe hepatic disease due to potential for increased sedation. |
| Pediatric use | For children 3-12 years: initial 10-15 mg/kg/day in three divided doses, titrate over 3 days to effective dose; usual maintenance 25-35 mg/kg/day in three divided doses. Maximum 50 mg/kg/day. For children 12 years and older: same as adult dosing. |
| Geriatric use | Initiate at lower doses (100-300 mg once daily) and titrate slowly due to age-related renal function decline. Adjust dose based on creatinine clearance. Monitor for sedation, dizziness, and ataxia. |
| 1st trimester | Associated with increased risk of major congenital malformations, particularly cardiac defects and neural tube defects; use only if benefit outweighs risk. |
| 2nd trimester | Potential for adverse fetal effects; limited data, but risk of malformations persists; use cautiously. |
| 3rd trimester | Risk of neonatal withdrawal syndrome (e.g., poor feeding, irritability) and osteomalacia; avoid near term unless essential. |
Clinical note
Antacids decrease absorption May cause somnolence dizziness and respiratory depression especially with opioids.
| Placental transfer | Gabapentin crosses the placenta; cord blood concentrations are similar to maternal plasma concentrations (ratio ~1.0). Transfer is via simple diffusion due to moderate lipophilicity. |
| Breastfeeding | Gabapentin is excreted into breast milk in low concentrations (milk:plasma ratio ~0.7-1.0). A study of 11 mothers reported relative infant dose of ~1.3% of weight-adjusted maternal dose. No adverse effects observed in breastfed infants, but monitor for sedation, poor feeding, and hypotonia. Use with caution, especially in preterm or compromised infants. |
■ FDA Black Box Warning
No FDA boxed warning.
| Common Effects | neuropathic pain |
| Serious Effects |
Hypersensitivity to gabapentin or any excipients
| Precautions | May cause dizziness, somnolence, and ataxia, Increased risk of suicidal thoughts or behavior, Respiratory depression when used with CNS depressants or in patients with respiratory risk factors, Abrupt discontinuation may precipitate withdrawal symptoms, May cause acute pancreatitis, Use caution in renal impairment; dose adjustment required |
| Food/Dietary | No significant food interactions. However, high-fat meals may alter absorption; take consistently with or without food. |
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| Lactation Rating | L2 (Moderately Safe) |
| Teratogenic Risk | First trimester: Limited data suggest a small increased risk of major congenital malformations (e.g., cardiac defects) based on some observational studies; risk appears lower than with older AEDs. Second/third trimester: No specific pattern of fetal harm; gabapentin crosses placenta. Peripartum: Neonatal withdrawal syndrome (irritability, feeding difficulties) reported with chronic exposure; monitor neonate. |
| Fetal Monitoring | Baseline and periodic maternal renal function (creatinine, eGFR) due to renal clearance. Serum gabapentin levels not routinely monitored; consider if efficacy/safety concerns. Fetal ultrasound for structural anomalies if exposed in first trimester. Neonatal observation for withdrawal symptoms (e.g., irritability, jitteriness, feeding problems) for 48 hours after birth. |
| Fertility Effects | No known adverse effects on human fertility. Animal studies show no impairment of fertility at clinically relevant doses. Gabapentin is not a strong enzyme inducer; unlikely to affect hormonal contraceptive efficacy. |
| Clinical Pearls | Gabapentin requires dose adjustment in renal impairment (CrCl <60 mL/min). Titrate slowly to avoid sedation and dizziness. Abrupt discontinuation may precipitate withdrawal symptoms. Note: Gabapentin has low oral bioavailability that decreases with increasing dose due to saturable absorption. Use with caution in elderly due to increased fall risk. |
| Patient Advice | Take gabapentin exactly as prescribed; do not stop suddenly without consulting your doctor. · Avoid driving or operating heavy machinery until you know how gabapentin affects you, as it may cause dizziness and drowsiness. · Report any unusual mood changes, depression, or suicidal thoughts immediately. · Store at room temperature away from moisture and heat. · Do not take antacids within 2 hours of gabapentin as they may reduce absorption. |