Metabolic acidosis is a disturbance in the body's acid-base balance that results in excessive acidity of the blood.  Metabolic acidosis is diagnosed by laboratory tests measuring the serum bicarbonate level in the blood to determine the presence and severity of metabolic acidosis.  

Metabolic acidosis can result in hyperventilation, and non-specific symptoms such as fatigue and anorexia, or more severe symptoms including cardiac arrhythmias or stupor.  Chronic metabolic acidosis can have adverse effects on the kidneys and on bones, and can retard growth in children.  Patients with predisposing conditions or therapies, including renal disease, severe respiratory disorders, diarrhea, surgery, ketogenic diet, or certain other drugs may be at greater risk for developing metabolic acidosis following treatment with zonisamide.  The risk of zonisamide-induced metabolic acidosis appears to be more frequent and severe in younger patients.     

The FDA recommends that healthcare professionals measure serum bicarbonate before starting treatment and periodically during treatment with zonisamide, even in the absence of symptoms.  If metabolic acidosis develops and persists, consideration should be given to reducing the dose or discontinuing zonisamide (using dose tapering), and modifying the patient’s antiepileptic treatment as appropriate.  If the decision is made to continue patients with metabolic acidosis on zonisamide, then alkali treatment should be considered. 

The FDA is working with the makers of zonisamide to revise the product labeling to reflect this new safety information.

This information reflects FDA's current analysis of data available to FDA concerning this drug. FDA intends to update this when additional information or analyses become available.