Mysoline

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What Is Mysoline?

Mysoline is an oral tablet containing primidone 250 mg. Primidone is a barbiturate-class anticonvulsant that has been in clinical use since 1954. It is metabolised in the liver to two active metabolites: phenobarbital (long-acting) and phenylethylmalonamide (PEMA).

Primidone has two distinct clinical niches: as an adjunctive anticonvulsant for drug-resistant focal and generalised tonic-clonic seizures, and as a first-line treatment for essential tremor (with efficacy comparable to propranolol). Mysoline is the original brand and remains the reference standard for primidone formulations.

Because primidone converts to phenobarbital in vivo, therapeutic drug monitoring should track both primidone (5–12 µg/mL) and phenobarbital (15–40 µg/mL) levels.

How Does Mysoline (Primidone) Work?

Primidone itself has anticonvulsant activity, but it is also extensively hepatically metabolised into two active metabolites:

• Phenobarbital — a long-acting barbiturate that enhances GABA_A receptor activity and prolongs chloride channel opening
• PEMA (phenylethylmalonamide) — has independent anticonvulsant properties

The combined action of the parent drug and its metabolites provides seizure control through multiple inhibitory mechanisms. For essential tremor, the mechanism is less well understood but may involve cerebellar and olivary nucleus suppression.

Indicaties

• Generalised tonic-clonic (grand mal) seizures
• Complex partial (focal) seizures
• Essentiële tremor — first-line alongside propranolol (AAN guideline)
• Benign familial tremor

Dosing and Administration

Epilepsy

Essential Tremor

Start at 25–50 mg at bedtime (lower than epilepsy). Increase by 25–50 mg every 1–2 weeks. Effective range: 250–750 mg/day. Many patients respond to 250 mg/day or less. Initial-dose side effects (sedation, dizziness, nausea) often resolve — starting very low reduces dropouts.

Bijwerkingen

Common (especially during initiation): sedation, dizziness, nausea/vomiting, ataxia, vertigo — often called the “primidone reaction.” These typically diminish within the first 1–2 weeks.

Langdurig gebruik: cognitive slowing, mood changes, sexual dysfunction, osteomalacia (via vitamin D metabolism — same mechanism as phenytoin/phenobarbital), megaloblastic anaemia (folate depletion).

Serious: blood dyscrasias (rare — agranulocytosis, aplastic anaemia), hepatotoxicity, SLE-like syndrome (very rare).

Waarschuwingen en voorzorgsmaatregelen

• Suicidal behaviour — class warning. All anticonvulsants carry an FDA warning for increased suicidal ideation. Monitor mood changes.
• Sedation — particularly severe on initiation. The first dose of primidone can cause dramatic sedation, dizziness, nausea and ataxia (“primidone vertigo”). Start with a bedtime dose of 50–125 mg and titrate very slowly over 2–4 weeks.
• Enzyme induction. Primidone/phenobarbital potently induces CYP3A4, CYP2C9, CYP2C19 and UGT enzymes. This reduces the efficacy of oral contraceptives, warfarin, corticosteroids, lamotrigine, and many other drugs. Use non-hormonal contraception or high-dose oestrogen formulations.
• Abrupt withdrawal. Sudden cessation can precipitate status epilepticus. Taper over 2–4 weeks minimum.
• Vitamin D depletion & osteoporosis. Chronic use accelerates vitamin D catabolism via enzyme induction. Supplement vitamin D 1,000–2,000 IU/day. Monitor DEXA scans in patients on >2 years of therapy.
• Folate depletion. Primidone depletes folate, raising homocysteine and megaloblastic anaemia risk. Supplement folate 1–5 mg/day, especially in women of childbearing age.
• Pregnancy — category D. Phenobarbital (active metabolite) is teratogenic. Fetal barbiturate syndrome includes craniofacial anomalies, growth restriction and neurodevelopmental delay. Neonatal withdrawal and bleeding (vitamin K depletion) are also risks. Use only if no alternative controls seizures.
• Hepatic impairment. Primidone is hepatically metabolised. Use lower doses and monitor levels in liver disease.
• Driving. Do not drive until you have been on a stable dose for 2+ weeks. Sedation is common.

Contraindications — Who Should NOT Take Mysoline

• Known hypersensitivity to primidone, phenobarbital or any excipient
• Porphyria (barbiturates are absolutely contraindicated)
• Severe respiratory insufficiency
• Patients with prior barbiturate dependence or abuse history (relative contraindication)

Geneesmiddelinteracties

• CNS depressants: Additive sedation (opioids, benzodiazepines, alcohol)
• CYP enzyme induction: Primidone (via phenobarbital) induces CYP3A4, CYP2C, and CYP1A2 — reduces levels of oral contraceptives, warfarin, corticosteroids, immunosuppressants, and many other drugs
• Valproate: Inhibits phenobarbital metabolism → toxicity risk; also displaces from protein binding
• Isoniazid: Inhibits primidone metabolism → increased levels
• Carbamazepine: Mutual induction — complex bidirectional interaction

Bewaaradvies

• Bewaren bij kamertemperatuur, 15–30°C. Bescherm tegen vocht.
• Keep in original packaging until use.
• Keep out of reach of children — primidone/phenobarbital overdose is life-threatening.
• Gebruik niet na de vervaldatum.

Related Categories on MedsBase

• Bekijk alle neurologische & epilepsiemedicijnen
• Bekijk medicijnen voor geestelijke gezondheid & psychiatrie

Veelgestelde vragen

What is Mysoline used for?

Mysoline contains primidone 250 mg and is used to control tonic-clonic and complex partial seizures in epilepsy. It is also a first-line treatment for essential tremor (hand and head tremor that worsens with movement).

Is primidone a barbiturate?

Primidone is a barbiturate derivative (deoxybarbiturate). It is not itself a barbiturate, but one of its two active metabolites — phenobarbital — is. This means primidone carries barbiturate-class warnings (dependence potential, withdrawal seizures, CNS depression).

How does Mysoline compare to propranolol for essential tremor?

Both are first-line for essential tremor (AAN/EFNS guidelines). Propranolol works better for limb tremor and has a more familiar side-effect profile. Primidone is more effective for some patients and can be combined with propranolol when monotherapy is insufficient. The choice depends on comorbidities, tolerability, and patient preference.

Why is starting Mysoline so unpleasant?

The “primidone reaction” (severe nausea, dizziness, sedation) occurs because the parent compound peaks before tolerance develops. Starting at 25–50 mg at bedtime (cutting the 250 mg tablet) dramatically reduces this reaction. Most patients tolerate the drug well after the first week.

Does Mysoline require blood level monitoring?

Both primidone and its metabolite phenobarbital can be measured. Phenobarbital levels (therapeutic: 15–40 µg/mL) are the more clinically useful guide for epilepsy, since phenobarbital is the long-acting active metabolite. For essential tremor, dosing is typically guided by clinical response rather than levels.

Can I stop Mysoline suddenly?

Nee. Because phenobarbital (a major metabolite) has a very long half-life, abrupt withdrawal after chronic use can cause rebound seizures or status epilepticus. Tapering should be gradual over weeks to months.

Does primidone affect oral contraceptives?

Yes — primidone (through phenobarbital) strongly induces CYP3A4, which accelerates the metabolism of oestrogen and progestogen. Hormonal contraceptives may fail. Use a non-hormonal method (copper IUD) or a method not affected by enzyme induction.

Is Mysoline safe during pregnancy?

Primidone is teratogenic (via phenobarbital metabolite). It is associated with congenital heart defects and facial abnormalities. Switch to a safer anticonvulsant before conception when possible. If primidone must be continued, high-dose folate and specialist monitoring are essential.

What is the difference between Mysoline and phenobarbital?

Primidone is a prodrug that is converted to phenobarbital plus PEMA. Some patients who do not respond to phenobarbital alone respond to primidone, suggesting the parent compound and PEMA contribute independently. Primidone may also have a faster onset of action for tremor because peak parent-drug levels occur before phenobarbital accumulates.

Can I drink alcohol while taking Mysoline?

Vermijd alcohol. Both primidone/phenobarbital and alcohol are CNS depressants — combined use greatly increases sedation, impaired coordination, respiratory depression risk, and cognitive impairment.

Does Mysoline cause weight gain?

Modest weight gain is possible with long-term use (barbiturate-class effect), though it is less common than with valproate or gabapentin. Sedation-related reduction in physical activity may contribute.

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