Gabapentin
Mechanism :
Gabapentin is structurally related to the neurotransmitter GABA (gamma-aminobutyric acid) but it does not modify GABA radioligand binding, it is not converted metabolically into GABA or a GABA agonist, and it is not an inhibitor of GABA uptake or degradation. The mechanism by which gabapentin exerts its analgesic action is unknown, but probably gabapentin prevents allodynia i.e. pain-related behaviour in response to a normally innocuous stimulus and hyperalgesia.
Indication :
- Partial seizure
- Post herpetic neuralgia (in adults)
- Neuropathic pain (in adults)
- Fibromyalgia (in adults)
Contraindications :
Gabapentin is contraindicated in patients who have demonstrated hypersensitivity to the drug or its ingredients and avoid abrupt withdrawal. Reduce dose in renal impairment.
Dosing :
Partial seizure:
Under 3 years:
Safety and efficacy not established.
3-4 years (Maintenance dose):
40 mg/kg/day orally in three divided doses.
5-12 years (Maintenance dose):
25-35 mg/kg/day orally in three divided doses.
Above 12 years (initial dose):
300 mg orally thrice a day; can be increased up to 600 mg orally thrice daily.
Post herpetic neuralgia:
Adults:
300-600 mg 3 times a day orally, start with 300 mg once day and then increase to twice a day next day and then 3 times a day.
Max dose: 1800 mg/day.
Neuropathic pain:
Adults:
300-1200 mg 3 times a day orally, start with 300 mg once day and then increase to twice a day next day and then 3 times a day.
Max dose: 3600 mg/day.
Fibromyalgia:
Adults:
300-900 mg 3 times a day orally, start with 300 mg once day and then increase to twice a day next day and then 3 times a day.
Max dose: 2400 mg/day.
Adverse Effect :
Ataxia, dizziness, drowsiness, fatigue, somnolence, nystagmus, diplopia, tremor, back pain, constipation, amblyopia, depression, dysarthria, dry mouth, hostility, hyperkinesia, leukopenia, peripheral edema.
Interaction :
Naproxen: Coadministration appears to increase the amount of gabapentin absorbed.
Hydrocodone: Coadministration decreases hydrocodone.
Cimetidine: Appeared to alter the renal excretion of both gabapentin and creatinine, an endogenous marker of renal function.
Renal Dose :
Dose in Renal Impairment GFR (mL/min)
| 30-60 | Start at low dose and increase dose according to response |
| 15-30 | Start at low dose and increase dose according to response |
| <15 | 300 mg on alternate days or 100 mg at night initially, increase according to tolerability |
Dose in Patients undergoing Renal Replacement Therapies
| CAPD | Probably dialysed. Dose as in GFR<15 mL/min. |
| HD | Dialysed. Loading dose of 300– 400 mg in patients who have never received gabapentin. Maintenance dose of 100–300 mg after each HD session and increase according to tolerability. |
| HDF/High flux | Dialysed. Loading dose of 300– 400 mg in patients who have never received gabapentin. Maintenance dose of 200–300 mg after each HD session and increase according to tolerability. |
| CAV/VVHD | Dialysed. Dose as in GFR=15– 30 mL/min |
Hepatic Dose :
No dosage adjustments are needed unless there is concomitant renal dysfunction.