Gabapentin (Neurontin, Pfizer)
Gabapentin was the second new antiepileptic medication released in the wave of new medicines in the mid 1990's. Gabapentin is useful for treatment of partial and secondarily generalized seizures. It is not effective for absence, atonic seizures or myoclonic seizures. A considerable amount of gabapentin is sold for non-epilepsy uses, including treatment of nerve pain, headache, mood stabilization, abnormal movements, and other conditions. All but epilepsy and pain are considered unofficial and unproven uses.
Advantages of gabapentin include its lack of drug interactions. It does not change levels of other seizure medications. It is cleared by the kidney, so it does not interact at the level of the liver. Gabapentin is normally very well-tolerated, occasionally producing dizziness, unsteadiness, sleepiness and uncommonly GI side effects, as well as rare other side effects. Patients may feel better on gabapentin than they do on other older antiepileptic medications. This may be particularly useful for the elderly, who are quite drug sensitive, and individuals who are on the verge of not wanting to be treated at all.
Disadvantages of gabapentin include the short half-life which requires a three times daily regimen. Gabapentin has a reputation for being less effective than other medications against partial seizures, although it clearly is effective. This reputation comes from a typical responder rate (the fraction of patients whose seizures are cut in half or better) in the range of 20-30%. In fairness to gabapentin, these studies were done at low doses of the medicine, in the range of 900-1800 mg, and as add-on therapy in cases of very hard-to-treat seizures. There is an unproven, but plausible belief that efficacy is better at doses up to 3600 or even 4800 mg of gabapentin per day. Serum levels are not very useful. Gabapentin often is used as an add-on medication to another seizure medicine.
Summary Data for Gabapentin
Pill sizes:
Capsules
100 mg (white)
300 mg (yellow)
400 mg (orange)
Tablets
600 mg (oval, tan, not scored)
800 mg (oval, white, not scored)
Liquid for oral: suspension 250 mg/5ml.
Injectable: none
Typical adult dose: 300 mg day one, 600 m day two, 900 mg day 3 divided into 3 doses. Can also start 900 mg divided into 3 doses. Maintenance is 900 - 4800 mg/d divided into 3 or 4 doses.
Typical pediatric dose: start with 10-15 mg/kg/d in 3 divided doses, increase as tolerated up to 60 mg/kg/d.
Metabolism: excreted unchanged in the kidney.
Half-life: 12-18 hours.
Serum levels: 2-20 mcg/L, not very useful.
Pregnancy: Category C - can cause birth defects in animals, unknown in humans.
Drugs that raise GPN levels: none
Drugs that lower GPN levels: antacids (minor effect)
GPN increases effects of: carbamazepine (increased dizziness through unknown action)
GPN decreases effects of other drugs: none known. GPN does not affect oral contraceptives.
Dangerous Side Effects
Very rare reduction of blood counts, possible birth defects.
Common Side Effects
Dizziness, unsteadiness, fatigue.
Other Side Effects
Cognitive problems, weight gain, GI problems, slurred speech, fluid retention and edema, muscle aches, mood changes.