Tablet
- Strengths
- 50, 250 mg
- Product note
- No commercial U.S. oral liquid identified. Any compounded suspension needs a pharmacy-specified concentration and stability.
Primidone
Mysoline
DailyMed-verified antiseizure indications
Indication / use
- Label / evidence summary
- Primidone: control of tonic-clonic (grand mal), psychomotor, and focal seizures, alone or with other anticonvulsants.
- Role
- Maintenance
- Class
- Converts to two active metabolites: phenobarbital and phenylethylmalonamide (PEMA)
Indications checked 26 September 2026. Product, route, age, and treatment-role limits apply. A DailyMed listing alone does not establish FDA approval.
- Primidone tablet U.S. label
Focal seizures, Generalized tonic-clonic seizures · Exact minimum age not mapped · MaintenanceLabel has pediatric schedules but no fixed minimum age.
Formulations & strengths
U.S. products unless another country is named. Strengths are per unit or stated volume.
Brand & formulation labels
Each link names the product and formulation it covers. Archived labels do not establish current availability.
Dosing & administration
- Adult / product-specific schedule
- Gradual titration to 750–1,500 mg/day divided.
- Pediatric
- Weight-based; label provides age-specific schedules.
- Titration / repeat dosing
- Adults and children ≥8 years: 100–125 mg at bedtime for days 1–3, twice daily days 4–6, three times daily days 7–9, then 250 mg three or four times daily as needed. Pediatric schedules vary by age.
Dose adjustment & concentrations
- Renal impairment
- Reduce/caution.
- Hepatic impairment
- Use caution.
- Serum reference information
- 5–12 mcg/mL primidone; phenobarbital 15–40 mcg/mL
Safety
- Boxed warning
- No boxed warning in standard U.S. labeling. Antiseizure-drug class suicidality warning applies.
- Contraindications
- Porphyria; hypersensitivity to phenobarbital.
- Serious precautions & monitoring
- Acute sedation/ataxia can be prominent at initiation; enzyme-inducing active metabolite.
Common / selected adverse effects
- Ataxia and vertigo frequently occur early
- Drowsiness, nausea, and diplopia reported; precise modern trial rates unavailable
Drug interactions: toxicity & clinical action
| Partner / combination | Clinical effect | Clinical action |
|---|---|---|
| Valproate | Can increase the active phenobarbital metabolite, with sedation/ataxia. | Check phenobarbital as well as primidone concentrations when clinically indicated. |
| Hormonal contraceptives / other CYP or UGT substrates | Primidone/phenobarbital enzyme induction lowers exposure. | Review contraceptive and partner-drug effectiveness, including after withdrawal. |
| Opioids, alcohol or benzodiazepines | Additive CNS/respiratory depression. | Monitor sedation and breathing; avoid unnecessary combinations. |
Pregnancy & contraception
- Fetal / neonatal risk
- Human primidone pregnancy risk is incompletely characterized. The label describes birth-defect concerns and neonatal bleeding with a vitamin-K-like coagulation defect after maternal anticonvulsant exposure.
- Contraception
- Primidone and its phenobarbital metabolite can reduce hormonal contraceptive effectiveness. The UK SmPC recommends highly effective contraception during treatment and for two months after stopping, with a method not compromised by enzyme induction.
Clinical evidence
Research summaries describe studied populations and outcomes; they do not add indications or constitute treatment recommendations.
Extrapolation
Trial summary
- Study design / duration
- Predates modern pivotal Phase 3 requirements; label evidence is based on older controlled clinical studies and accumulated clinical use.
- Primary endpoint
- No single pivotal endpoint is identified in the cited source.
- Results
- Approval predates modern Phase 3 standards. Older active-controlled studies and clinical experience established efficacy across focal and generalized tonic-clonic seizures.
Trial publications
Mechanism of action
- Converts to two active metabolites: phenobarbital and phenylethylmalonamide (PEMA)
Pharmacology
Adult values unless specified. Vd/F denotes apparent oral distribution volume.
Pharmacokinetics
- Bioavailability
- Rapidly absorbed orally; a numeric absolute bioavailability is not specified in the cited UK SmPC.
- Elimination half-life
- Approximately 10 hours for primidone (UK SmPC); its active metabolites persist longer.
- Volume of distribution
- Approximately 0.6 L/kg in adults (NHS laboratory reference).
- Active metabolite(s)
- Yes: phenobarbital and phenylethylmalonamide (PEMA).
- Active-metabolite half-life
- Phenobarbital: 50–160 hours; PEMA: 10–25 hours (primidone UK SmPC). Both depend on age, organ function and interacting therapy.
- Protein binding
- Approximately 35% for primidone (UK SmPC).
- Metabolism / elimination
- Hepatic conversion to phenobarbital and PEMA
Measure and interpret primidone and phenobarbital separately when monitoring is indicated; their half-lives differ markedly.
Molecular structure
NIH PubChem structure and record ↗
Product history
- Initial U.S. approval
- 1954
- Brands
- Mysoline
- Manufacturer / marketer
- Bausch Health/Valeant legacy brand (Mysoline)
- Generic availability
- Yes
- Related drugs
- Metabolized to phenobarbital; closely related clinically to barbiturates.
Sources & review status
Representative sources are selected product labels, not an exhaustive list of generic manufacturers. Consult the exact product and formulation prescribed. Brand-name package inserts are linked separately.
Brand reference: Mysoline ↗
Indications and DailyMed PDFs checked: 2026-09-26. Other profile sources reviewed: 2026-09-15 · label revision noted at that review: 5/2025. This is a draft reference; final review is pending.