Felbamate dosage

In spite of the seriousness of the adverse effects, thousands of patients worldwide remain on the medication. Usual dosages are 2000-4000 mg/d in adults, and effective plasma levels range from 30 mcg/mL to 100 mcg/mL. In addition to its usefulness in partial seizures, felbamate has proved effective against the seizures that occur in Lennox-Gastaut syndrome. 

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. 

An established interaction. If felbamate is added to established treatment with phenobarbital or primidone, particularly in patients already taking substmtial doses, monitor well for any evidence of increased adverse effects (drowsiness, lethargy, anorexia, ataxia) and reduce the dosages of the phenobarbital or primidone if necessary. 

It was suggested that the vaccine inhibits the liver enzymes concerned with the metabolism of phenobarbital, thereby reducing its loss from the body. Information is very limited. Note that, a similar 30% increase in phenobarbital levels with felbamate has eventually required a dosage adjustment however with this interaction the increase will eventually be self-limiting. Therefore it seems unlikely that this moderate increase in phenobarbital levels will be of clinical significance.. 

A pilot study in 4 patients noted that felbamate increased plasma phenytoin levels. Therefore, in a further study the phenytoin dose was automatically reduced by 20% when felbamate was given. Of 5 patients, one needed a slight increase in phenytoin dosage, whereas 2 others needed a further reduction in their phenytoin dosage. In a later full report of this study, it was noted that phenytoin dosage decreases of 10 to 30% were required to maintain stable levels. Another study in epileptic patients found that felbamate 1.2 or 1.8 g daily increased the maximum plasma phenytoin levels by 31% and 69%, respectively. Higher felbamate doses necessitated phenytoin dose reductions of 20 to 40%. ... 

Established interactions. The phenytoin dosage may need to be reduced (a 20 to 40% reduction seems to be about right / ) if felbamate is added, and to increase it if felbamate is withdrawn. However, note that as phenytoin pharmacokinetics are non-linear any dosage adjustments will need to be assessed in individual patients. The importance of the reduced felbamate levels is uncertain, but is probably less important because felbamate has a wide therapeutic range. ... 

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