MYSOLINE
Clinical safety rating: caution
Comprehensive clinical and safety monograph for MYSOLINE (MYSOLINE).
Primidone is a barbiturate anticonvulsant that acts by enhancing GABA-A receptor activity and possibly by blocking sodium channels.
| Metabolism | Primidone is metabolized primarily by the liver via CYP2C9 and CYP2E1 to two active metabolites: phenobarbital and phenylethylmalonamide. |
| Excretion | Primidone is excreted primarily in urine; approximately 60-80% as unchanged drug and metabolites (PEMA, phenobarbital), with less than 10% in feces. |
| Half-life | Primidone: 5-15 hours (mean 10 hours); PEMA: 10-18 hours; Phenobarbital: 50-120 hours. Steady state achieved in 2-4 weeks due to accumulation of phenobarbital. |
| Protein binding | Primidone: 20-30%; phenobarbital: 45-60%. Primidone binds to plasma proteins, mainly albumin. |
| Volume of Distribution | Primidone: 0.6-1.0 L/kg (approx 42-70 L in 70 kg adult), indicating distribution into total body water. |
| Bioavailability | Oral: 90-100%; food does not significantly affect absorption. |
| Onset of Action | Oral: 30-90 minutes for peak effect; antiepileptic effect may take several days to weeks for full therapeutic benefit. |
| Duration of Action | 6-12 hours for primidone itself, but due to active metabolites (especially phenobarbital), effect persists for 24-48 hours, allowing twice-daily dosing. |
250 mg orally 3 times daily; may increase by 250 mg/day every 3 days; usual maintenance 250 mg 3-4 times daily; maximum daily dose 1500 mg.
| Dosage form | TABLET |
| Renal impairment | CrCl <50 mL/min: not recommended; CrCl 50-80 mL/min: administer every 12 hours; CrCl >80 mL/min: usual dosing. |
| Liver impairment | Child-Pugh A: no adjustment; Child-Pugh B: reduce dose by 25-50%; Child-Pugh C: contraindicated. |
| Pediatric use | >8 years: 10-20 mg/kg/day in 2-3 divided doses; <8 years: 10-15 mg/kg/day in 2-3 divided doses; maximum 1500 mg/day. |
| Geriatric use | Start at 250 mg once daily; increase slowly; monitor for sedation and ataxia. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for MYSOLINE (MYSOLINE).
| Breastfeeding | Primidone enters breast milk; M/P ratio approximately 0.6-1.0. Infant serum levels can reach therapeutic ranges. Monitor for sedation, irritability, and poor feeding. Use with caution; avoid if possible in breastfeeding. |
| Teratogenic Risk | First trimester: Increased risk of major congenital malformations including orofacial clefts, cardiac defects, and neural tube defects (3-5x baseline). Second and third trimesters: Risk of neonatal hemorrhage (vitamin K-dependent clotting factors), hypocalcemia, and withdrawal symptoms. Chronic exposure: Possible neurodevelopmental delays. |
■ FDA Black Box Warning
Barbiturates are habit-forming. Tolerance, psychological and physical dependence may occur. Withdrawal symptoms (including seizures, hallucinations, and death) may occur after abrupt discontinuation. Use in pregnancy may cause fetal harm.
| Serious Effects |
Absolute: hypersensitivity to primidone, barbiturates, or any component; porphyria; respiratory depression; severe hepatic impairment.
| Precautions | May cause somnolence, dizziness, and ataxia; avoid abrupt withdrawal; monitor for hypersensitivity reactions; use with caution in hepatic or renal impairment; may exacerbate porphyria. |
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| Fetal Monitoring |
| Preconception: Folic acid supplementation (4-5 mg/day). First trimester: Ultrasound for structural anomalies. Second trimester: Fetal echocardiogram and detailed anatomy scan. Third trimester: INR/coagulation profile monthly; vitamin K1 10 mg/day orally starting at 36 weeks. Neonatal: Vitamin K injection at birth; monitor for withdrawal and bleeding. |
| Fertility Effects | May reduce fertility due to alterations in sex hormone metabolism (increased clearance of estrogens/progestins) and menstrual irregularities. No permanent impairment documented. |