Dosing and uses of Mysoline (primidone)
Adult dosage forms and strengths
tablets
- 50mg
- 250mg
Seizures (Psychomotor, Focal, & Grand Mal)
Initial: 100-125 mg PO qHS for 3 days, THEn
100-125 mg BID for 3 days, THEn
100-125 mg TID for 3 days, THEn
250 mg TID-QID; not to exceed 2 g/day
Dosing considerations
- May take with food
- Monitor: CBC, SMA-12 q6mo
- Do not exceed 2 g/day
- May control grand mal seizures refractory to other anticonvulsants
- Do not abruptly discontinue, due to risk of precipitation of status epilepticus
- Therapeutic efficacy of dosage regimen may take several weeks to assess
Essential Tremor
12.5-25 mg/day PO Hs
Increase dose over 2-3 weeks to 250 mg PO qDay or divided BID; do not exceed 750 mg/day
Partial Seizures
125 mg PO QHs
Increase dose every 3 days by 125 mg if needed to 250 mg q12hr; do not exceed 500 mg/day
Pediatric dosage forms and strengths
tablets
- 50mg
- 250mg
Seizures (Psychomotor, Focal, & Grand Mal)
Initial: 50 mg PO qHS for 3 days, THEn
50 mg BID for 3 days, THEn
100 mg BID for 3 days, THEn
After 9th day: 125-250 mg PO TID, OR 10-25 mg/kg/day divided q8hr PO
Dosing Considerations
May take with food
Monitor: CBC, SMA-12 q6mo
Do not exceed 2 g/day
May control grand mal seizures refractory to other anticonvulsants
Do not abruptly discontinue, due to risk of precipitation of status epilepticus
Therapeutic efficacy of dosage regimen may take several weeks to assess
Mysoline (primidone) adverse (side) effects
1-10%
Dysarthria
Paresthesia
Ataxia
Vertigo
Pediatric patients
- Paradoxical excitement
- Hyperactivity
Geriatric patients
- Excitement
- Confusion
- Depression
Adverse effects that decrease with continued treatment
- Drowsiness
- Ataxia
- Irritability
- Headache
- Restlessness
- Nystagmus
- Dizziness
- Vertigo
Frequency not defined
Acute psychosis (rare)
Nausea
Vomiting
Constipation
Diarrhea
Megaloblastic (folate-deficiency) anemia
Hepatotoxicity
Hypocalcemia
Rickets (rare)
Osteomalacia (rare)
Rash
Stevens-Johnson syndrome (rare)
Warnings
Contraindications
Porphyria
Hypersensitivity to phenobarbitaL
Cautions
Do not interchange brands, due to bioequivalence issues
May cause hyperexcitability, especially in children
When replacing another antiseizure drug, gradually increase dosage of primidone while gradually decreasing dosage of other drug over at least 2 weeks
Withdraw gradually, due to risk of precipitation of status epilepticus
Limited number of reports indicate maintaining serum concentration of 5-12 mcg/mL is necessary
Inhibits transplacental vitamin K transport, leading to increased risk of fetal hemorrhage
May render OCPs ineffective
Pregnancy and lactation
Pregnancy category: D; continue use if pregnant; consider vitamin K supplementation for 1 month before birth
Lactation: Distributed in breast milk; discontinue if drug effects occur in nursing infant
Pregnancy categories
A: Generally acceptable. Controlled studies in pregnant women show no evidence of fetal risk.
B: May be acceptable. Either animal studies show no risk but human studies not available or animal studies showed minor risks and human studies done and showed no risk.
C: Use with caution if benefits outweigh risks. Animal studies show risk and human studies not available or neither animal nor human studies done.
D: Use in LIFE-THREATENING emergencies when no safer drug available. Positive evidence of human fetal risk.
X: Do not use in pregnancy. Risks involved outweigh potential benefits. Safer alternatives exist.
NA: Information not available.
Pharmacology of Mysoline (primidone)
Mechanism of action
Metabolized to phenobarbitaL
Absorption
Bioavailability: 60-80%
Peak plasma time: 4 hr
Metabolism
Metabolized by liver
Metabolites: Phenobarbital, phenylethylmalonamide
Enzymes induced: CYP3A4
Elimination
Half-life: 10-12 hr; 24-48 hr (metabolites)
Dialyzable: Yes
Excretion: Urine (15-25%)
