The recently available intravenous formulation of valproic acid has several advantages:
It has not been compared directly to phenytoin, fosphenytoin, or phenobarbital, however, and may not be as effective in GCSE.
Valproic acid is administered in a loading dose of 25 mg/kg. Higher doses are occasionally used. The package insert recommends an infusion rate of no faster than 20 mg/min, but several reports have documented that infusion rates between 100 and 150 mg/min are safe and well tolerated.
Valproic acid may significantly increase phenobarbital levels or free phenytoin levels if these medications have already been administered.
Caution should be exercised if the patient is being treated with lamotrigine (Lamictal, a widely used maintenance anticonvulsant) because administration of valproate may quickly double lamotrigine levels. It is prudent to hold lamotrigine doses in this situation and check serum levels within 24 hours.
Rectally administered valproic acid syrup is absorbed irregularly and is rarely used now that an intravenous formulation is widely available. It can be useful if intravenous access can not be obtained, however, because it is absorbed more rapidly than valproic acid given by mouth and bypasses the hepatic first-pass effect. The usual dose is 25 mg/kg or higher. Valproic acid syrup is cathartic. It should be mixed in equal volumes of water prior to administration.
Reviewed and revised February 2004 by Thaddeus Walczak, MD, MINCEP® Epilepsy Care, Minneapolis, MN
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