Concurrent use is usually uneventful. Initially total
serum phenytoin levels may fall by 20 to 50% but this is offset by a rise in the levels of free (and active)
phenytoin, which may very occasionally cause some
toxicity. After continued use the total
serum phenytoin levels rise once again.
Phenytoin also reduces
valproate levels. Fospheny- toin, a prodrug of
phenytoin, may interact similarly.
Monitor
phenytoin and adjust the dose accordingly. When monitoring concurrent use it is important to understand fully the implications of changes in 'total', and 'free' or 'unbound'
serum phenytoin levels. Where monitoring of free
phenytoin levels is not available, various nomograms have been designed for predicting unbound
phenytoin levels during the use of sodium
valproate. Consider moni- toring
valproate levels for an indication of
toxicity if
phenytoin is stopped.