ZONISAMIDE
Clinical safety rating: caution
Animal studies have proved adverse effects but may be safe for humans
Anticonvulsant; blocks voltage-gated sodium channels and T-type calcium channels, reducing neuronal excitability and seizure propagation. Also weakly inhibits carbonic anhydrase.
| Metabolism | Hepatic; primarily via CYP3A4-mediated metabolism, with minor contribution from CYP2C19 and CYP2C9. Also undergoes acetylation. |
| Excretion | Renal: approximately 30% unchanged; remainder as glucuronide conjugate and reduced metabolite. Biliary/fecal: minimal (<5%). |
| Half-life | Terminal half-life approximately 60-70 hours (range 50-80 hours) in adults; at steady state, half-life may be slightly longer. Clinical context: requires 2-3 weeks to achieve steady state. |
| Protein binding | Approximately 40-60% bound to plasma proteins, primarily albumin. |
| Volume of Distribution | Approximately 1.1-1.7 L/kg, indicating extensive tissue distribution. |
| Bioavailability | Oral: approximately 100% (well absorbed); no significant first-pass metabolism. |
| Onset of Action | Oral: peak plasma concentrations at 2-6 hours; clinical effects (seizure control) typically observed within 1-2 weeks of initiating therapy, but full effect may take longer. |
| Duration of Action | Approximately 24 hours due to long half-life, allowing once- or twice-daily dosing. Clinical notes: steady-state levels achieved after 2-3 weeks. |
Oral, initial 100 mg daily, may increase by 100 mg every 2 weeks; maintenance 200-400 mg daily in 1-2 divided doses; maximum 600 mg daily.
| Dosage form | CAPSULE |
| Renal impairment | GFR 50-79 mL/min: no adjustment; GFR 30-49 mL/min: reduce dose by 50%; GFR <30 mL/min: contraindicated; hemodialysis: supplement 50% of dose post-dialysis. |
| Liver impairment | Child-Pugh A: no adjustment; Child-Pugh B: reduce dose by 50%; Child-Pugh C: contraindicated. |
| Pediatric use | Age 6-16 years: oral 1-2 mg/kg once daily initially, titrate by 1-2 mg/kg/day every 2 weeks; maintenance 4-8 mg/kg/day divided twice daily; maximum 12 mg/kg/day or 600 mg/day. |
| Geriatric use | Start at 50-100 mg daily; titrate slowly due to reduced renal function; monitor for somnolence and dizziness. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Strong CYP3A4 inducers may decrease levels Can cause oligohydrosis and hyperthermia in children and metabolic acidosis.
| Breastfeeding | Zonisamide is excreted into human breast milk. The milk-to-plasma ratio (M/P) is approximately 0.8–1.0. Infant serum concentrations can reach 11–36% of maternal levels, potentially causing sedation, poor feeding, or metabolic acidosis. Breastfeeding is generally not recommended due to risk of adverse effects; if used, monitor infant for somnolence, weight loss, and developmental milestones. |
| Teratogenic Risk | Zonisamide is classified as FDA Pregnancy Category C. In the first trimester, there is an increased risk of major congenital malformations, including neural tube defects, cleft lip/palate, and cardiac anomalies, possibly dose-related. Second and third trimester exposure may cause fetal growth restriction, low birth weight, and potential neurodevelopmental effects. Neonatal withdrawal syndrome (irritability, feeding difficulties) can occur with third trimester exposure. Risk is higher with polytherapy and doses >300 mg/day. |
■ FDA Black Box Warning
None.
| Common Effects | Somnolence |
| Serious Effects |
Hypersensitivity to zonisamide or sulfonamides (zonisamide is a sulfonamide derivative).
| Precautions | Serious skin reactions (e.g., Stevens-Johnson syndrome, toxic epidermal necrolysis), hypersensitivity, acute myopia and secondary angle-closure glaucoma, metabolic acidosis, oligohidrosis and hyperthermia (especially in children), suicidal behavior and ideation, withdrawal seizures if abruptly discontinued. |
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| Fetal Monitoring | Maternal: serum zonisamide concentrations (therapeutic range 10–40 mg/L), liver function tests, renal function, complete blood count, serum bicarbonate (monitor for metabolic acidosis), and signs of hypersensitivity. Fetal: prenatal ultrasound for structural anomalies (e.g., neural tube defects), fetal growth monitoring in third trimester. Neonatal: observe for withdrawal symptoms, metabolic acidosis, and coagulopathy (Vitamin K administered to neonate). |
| Fertility Effects | Zonisamide may impair fertility in males and females. In females, menstrual irregularities and anovulation have been reported, possibly due to enzyme induction affecting sex hormones. In males, reversible decreases in sperm motility and count have been observed in animal studies; human data limited. Use effective contraception in women of childbearing potential. |