Drug-Drug Interaction

VALPROIC ACID/HYDANTOINS

MECHANISM OF ACTION
Multiple mechanisms appear to be involved.

DISCUSSION
The outcome of this interaction in individual patients is variable. In most patients the active form of phenytoin (the unbound drug) is not significantly changed by valproic acid. Decreased serum phenytoin concentrations have been reported, as have symptoms of phenytoin toxicity. Increases in frequency of seizures have also been reported. Patients receiving phenytoin and valproic acid concurrently tend to have lower serum valproic acid concentrations than patients taking valproic acid alone.

CLINICAL EFFECTS
The pharmacological and toxic effects of phenytoin may be increased while the pharmacological effects of valproic acid may be decreased.

SEVERITY LEVEL
3-Moderate Interaction: Assess the risk to the patient and take action as needed.

PATIENT MANAGEMENT
Monitor serum phenytoin and valproic acid concentrations. Observe the patient for loss of seizure control and symptoms of phenytoin toxicity. Adjust the dose of either agent as needed.

PREDISPOSING FACTORS
None determined.

REFERENCES

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