Gabapentin dosage

Gabapentin-induced myoclonus may be more common than initially thought. In a retrospective survey, 13 of 104 consecutive patients developed myoclonus at gabapentin dosages of 800-3200 mg/day (11). Six of the patients had severe chronic static encephalopathy. Three had focal myoclonus, contralateral to the epileptic focus, and two had exacerbation of pre-existent myoclonus. In aU cases the myoclonus was mild and did not interfere with daily activities. The fact that in three patients the electroencephalogram did not show epileptiform discharges suggests that, at least in some cases, the myoclonus is non-epileptic in nature. 

Drug interactions are infrequent with gabapentin. It does not induce hepatic metabolizing enzymes, nor do other AEDs affect its metabolism and elimination. Antacids may decrease absorption. Gabapentin dosage may need to be decreased in patients with renal disease or in the elderly. 

Table 71.1. Gabapentin dosage based on renal function...

Gabapentin [Neurontin] Adult initially, 300 mg TID. Can be gradually increased up to 3600 mg/d based on desired response. Children (3-12 years of age] Initially, 10-15 mg/kg body weight in 3 divided dosages increase over 3 days until desired effect or a maximum of 50 mg/kg/d. Developed originally as an anticonvulsant may also be helpful as an adjunct to other drugs in treating spasticity associated with spinal cord injury and multiple sclerosis. 

Gabapentin is effective as an adjunct against partial seizures and generalized tonic-clonic seizures at dosages that range up to 2400 mg/d in controlled clinical trials. Open follow-on studies permitted... 

The efficacy of gabapentin in dosages up to 3600 mg/day as adjunctive therapy has been studied in 2016 patients with partial seizures (3). The four most commonly reported adverse events were somnolence (15%), dizziness (10%), weakness (5.8%), and headache (4.5%). 

In an uncontrolled trial using dosages up to 6000 mg/day in 50 patients with refractory partial epilepsy, tiredness, dizziness, headache, and diplopia were the most common adverse effects (4). At dosages above 3600 mg/day three patients developed flatulence and diarrhea and two had myoclonic jerks. At least in some patients, gabapentin gastrointestinal absorption did not become saturated within the explored dosage range. 

Gabapentin-induced anorgasmia and reduced libido have been reported in two women (38). Both were taking relatively low doses of gabapentin (900-1800 mg). In one case the symptoms disappeared with dosage reduction but in the other gabapentin had to be withdrawn. 

Because gabapentin is eliminated exclusively by the kidneys, dosage adjustments will be necessary in patients with significantly impaired renal function. In anuric patients, 35% of gabapentin is removed by dialysis. ... 

All anticonvulsants (except gabapentin), atypical antipsychotics, benzodiazepines, and calcium channel blockers require liver metabolism, and dosage adjustments may be needed (e.g., 25-50% reduction of normal doses) Carbamazepine or oxcarbazepine Alternative lamotrigine Acute mania or mixed episode first choice lithium... 

Tablets 600, 800 mg Dosage should be slowly or mixed episodes for gabapentin is structurally...

Gabapentin does not normally affect the pharmacokinetics of car-bamazepine, phenytoin, phenobarbital or sodium valproate, and no dosage adjustments are needed on concurrent use. However, isolated reports describe increased phenytoin levels and toxicity in two patients given gabapentin. 

It would seem therefore that no dosage adjustments are normally needed if gabapentin is added to treatment with most of these antiepileptics. However, if gabapentin is added to pbenytoin it may be wise to bear the possibility of raised pbenytoin levels in mind. For mention that gabapentin may prolong the half-life of felbamate, see Felbamate + Gabapentin , p.540. For mention of the lack of interaction between levetiracetam and gabapentin, see Levetiracetam + Other antiepileptics , p.543. 

Furthermore, evidence from clinical studies suggests that levetiraeetam does not affect the serum levels of gabapentin, lamotrigine, phenobarbital, or primidone. In general therefore, no dosage adjustments would seem to be needed if levetiraeetam is used as add-on therapy with any of these drugs. 

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