gabapentin
Neurontin

Available forms
Available by prescription only
Capsules: 100 mg, 300 mg, 400 mg
Oral solution: 250 mg/5 ml
Tablets: 600 mg, 800 mg

Indications and dosages
 Adjunctive treatment of partial seizures with and without secondary generalization. Adults and children age 13 and older: 300 mg P.O. on day 1, then 300 mg P.O. b.i.d. on day 2, and 300 mg P.O. t.i.d. on day 3. Increased as needed and tolerated to 1,800 mg daily, given in three divided doses. Usual dosage is 300 to 600 mg P.O. t.i.d., although doses up to 3,600 mg daily have been well tolerated.
Children ages 3 to 12: 10 to 15 mg/kg daily P.O. in three divided doses. Effective dose in children age 5 and older is 25 to 35 mg/kg daily P.O. in three divided doses. Effective dose in children ages 3 to 4 is 40 mg/kg daily P.O. in three divided doses. Doses up to 50 mg/kg daily have been well tolerated.
 Postherpetic neuralgia. Adults: 300 mg P.O. once daily on day 1, then 300 mg P.O. b.i.d. on day 2, then 300 mg P.O. t.i.d. thereafter. Adjust as needed to a maximum daily dose of 1,800 mg divided into three doses.
≡ Dosage adjustment. For patients age 12 and older with compromised renal function and patients receiving hemodialysis, use the dosage guide at the top of the next column. For patients with creatinine clearance of less than 15 ml/minute, reduce daily dose in proportion to creatinine clearance (for example, patients with a creatinine clearance of 7.5 ml/ minute should receive half the daily dose received by patients with a creatinine clearance of 15 ml/ minute). Patients on hemodialysis should receive maintenance doses based on estimates of creatinine clearance as indicated below and a supplemental posthemodialysis dose of 125 to 350 mg administered after each 4 hours of hemodialysis.

Creatinine clearance (ml/min) Total daily dose range (mg/day) ≥ 60 900-3,600 divided into 3 equal doses 30-59 400-1,400 divided into 2 equal doses 15-29 200-700 in 1 dose < 15 100-300 in 1 dose

Pharmacodynamics
Anticonvulsant action: Mechanism of action is unknown. Although it’s structurally related to gamma-aminobutyric acid (GABA), drug doesn’t interact with GABA receptors, isn’t converted metabolically into GABA or a GABA agonist, and doesn’t inhibit GABA uptake or degradation. Gabapentin exhibits no affinity for other common receptor sites.

Pharmacokinetics
Absorption: Bioavailability isn’t dose-proportional. A 400-mg dose, for example, is about 25% less bioavailable than a 100-mg dose. Over the recommended dose range of 300 to 600 mg t.i.d., however, differences in bioavailability aren’t large, and bioavailability is about 60%. Food may have slight effect on the rate or extent of absorption.
Distribution: Drug circulates largely unbound (less than 3%) to plasma protein. It crosses the blood-brain barrier with about 20% of the corresponding plasma levels found in CSF.
Metabolism: Not appreciably metabolized in humans.
Excretion: Eliminated from systemic circulation by renal excretion as unchanged drug. Elimination half-life is 5 to 7 hours. Drug can be removed from plasma by hemodialysis.

Contraindications and precautions
Contraindicated in patients hypersensitive to drug. Use cautiously in patients with altered renal function caused by drug accumulation.

Interactions
Drug-drug. Antacids: Decreases gabapentin absorption. Separate administration by at least 2 hours.

Adverse reactions
CNS: fatigue, somnolence, dizziness, ataxia, tremor, nervousness, dysarthria, amnesia, depression, abnormal thinking, twitching, incoordination.
CV: peripheral edema, vasodilation.
EENT: diplopia, rhinitis, pharyngitis, dry throat, coughing, dental abnormalities, amblyopia, nystagmus.
GI: nausea, vomiting, dyspepsia, dry mouth, constipation, increased appetite.
GU: impotence.
Hematologic: leukopenia.
Metabolic: weight gain.
Musculoskeletal: back pain, myalgia, fractures.
Skin: pruritus, abrasion.

Effects on lab test results
• May decrease WBC count.

Overdose and treatment
Acute gabapentin overdose may cause double vision, slurred speech, drowsiness, lethargy, and diarrhea.
 Supportive care is recommended. Gabapentin can be removed by hemodialysis and may be indicated by the patient’s clinical state or by presence of significant renal impairment.

Special considerations
• Don’t withdraw other anticonvulsants suddenly in patients starting gabapentin therapy. Discontinue drug therapy or substitute alternative drug gradually over at least 1 week to minimize risk of seizures.
• Drug can be taken without regard to meals.
 ALERT Don’t confuse Neurontin with Noroxin (norfloxacin).
• Routine monitoring of plasma drug levels isn’t needed. Drug doesn’t appear to alter plasma levels of other anticonvulsants.
Pregnant patients
• Data concerning use during pregnancy are inadequate.
Breast-feeding patients
• Drug appears in breast milk following oral administration. Women should discontinue breast-feeding because of potential for serious adverse reactions.
Pediatric patients
• Safety and efficacy in children younger than age 3 haven’t been established.

Patient education
• Instruct patient to take first dose at bedtime to minimize effects of drowsiness, dizziness, fatigue, and ataxia.
• Warn patient to avoid driving or operating heavy machinery until adverse CNS effects of drug are known.
• Inform patient that drug can be taken without regard to meals.

Reactions may be common, uncommon, life-threatening, or COMMON AND LIFE THREATENING.
◆ Canada only
◇ Unlabeled clinical use