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. |
| Molecular Weight | 218.25 |
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 | Teratogenic: Increased risk of major malformations including congenital heart defects, cleft lip/palate, and neural tube defects. Use only if benefit justifies risk. |
| 2nd trimester | Risk of bleeding due to vitamin K deficiency and coagulopathy. Monitor fetal growth; may cause fetal anticonvulsant syndrome. |
| 3rd trimester | Risk of neonatal hemorrhage, withdrawal syndrome, and respiratory depression. Administer vitamin K prophylaxis to neonate. |
Clinical note
Comprehensive clinical and safety monograph for MYSOLINE (MYSOLINE).
| Placental transfer | Complete and passive: primidone and its active metabolite phenobarbital readily cross the placenta, reaching equal or higher concentrations in fetal than maternal plasma (fetal-to-maternal ratio ~1). |
| Breastfeeding | Primidone and its metabolites are excreted in breast milk; infant serum levels may reach therapeutic range. Monitor for sedation, poor feeding, or unusual bleeding. Benefits of breastfeeding often outweigh low risk; consider safety profile of monotherapy at lowest effective dose. |
■ 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 |
Hypersensitivity to primidone or barbituratesAcute intermittent porphyriaSevere hepatic impairmentPorphyria variegataSevere respiratory insufficiency
| 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. |
| Food/Dietary | No specific food interactions are documented. However, alcohol consumption is contraindicated due to additive CNS depression and increased seizure risk. |
Loading safety data…
| Lactation Rating | L3 (Moderately Safe) - Limited data suggest risk but may be acceptable in some situations. |
| 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. |
| 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. |
| Clinical Pearls | MYSOLINE (primidone) is a barbiturate anticonvulsant metabolized to phenobarbital and PEMA. Its therapeutic drug monitoring should measure phenobarbital levels (target 15-40 mcg/mL). Avoid in porphyria and pregnancy. Monitor for sedation, ataxia, and nystagmus. Coadministration with valproate increases phenobarbital levels; adjust dose accordingly. Taper slowly to prevent withdrawal seizures. |
| Patient Advice | Take exactly as prescribed; do not stop suddenly without medical advice as withdrawal seizures may occur. · May cause drowsiness or dizziness; avoid driving or operating machinery until you know how this medication affects you. · Avoid alcohol as it can increase sedation and risk of seizures. · Use effective contraception during treatment; inform your doctor if you become pregnant or plan to become pregnant. · Report any rash, fever, or unusual bruising/bleeding to your healthcare provider immediately. · Take with food or milk if gastrointestinal upset occurs. · Keep all appointments for blood tests to monitor medication levels. |