Primidone, brand name Mysoline, is an oral anticonvulsant (anti-seizure) medication used for treating several types of seizures. Its exact mechanism of action is not known. Primidone is converted into phenobarbital and phenylethylmalonamide (PEMA) in the body. Primidone, phenobarbital, and PEMA all have anticonvulsant activity. In addition to its anticonvulsant activity, PEMA increases the anticonvulsant activity of phenobarbital. The FDA approved primidone in March, 1954.

Primidone is used alone or with other medications for treating grand mal, psychomotor, or focal epileptic seizures. It may be effective for grand mal seizures that are not responsive to other anticonvulsants. Primidone should not be used by patients with porphyria and people who are allergic to phenobarbital. It also is used for treating partial seizures and essential tremor.

Common side effects of primidone include:

• Difficulty speaking (dysarthria)
• Burning, tingling from damaged nerves (paresthesia)
• Difficulty moving (ataxia)
• Loss of balance (vertigo)
• Paradoxical excitement (children)
• Hyperactivity (children)
• Excitement
• Confusion
• Depression
• Drowsiness
• Irritability
• Headache
• Restlessness
• Nystagmus
• Dizziness

Possible serious side effects of primidone include:

• Acute psychosis
• Nausea
• Vomiting
• Constipation
• Diarrhea
• Folate-deficiency anemia
• Liver toxicity
• Low blood calcium
• Rickets
• Weakening of bones
• Rash
• Serious skin reactions (for example, Stevens-Johnson syndrome)

Seizure medications increase the risk of suicidal thoughts or behavior. Patients treated with seizure medications should be observed for the emergence or worsening of depression, suicidal thoughts or behavior, and unusual changes in mood or behavior.

Suddenly stopping medications for seizures may cause seizures.

To treat psychomotor, focal, or grand mal seizures

• Patients 8 years of age or older should be treated with the following regimen:
• Days 1 to 3: 100 to 125 mg at bedtime.
• Days 4 to 6: 100 to 125 mg twice daily.
• Days 7 to 9: 100 to 125 mg three times daily.
• Day 10 to maintenance: 250 mg three or four times daily.
• The maximum dose is 2 g daily.
• It takes several weeks before the antiseizure effects of primidone are seen.

To treat partial seizures

• The initial dose is 125 mg at bedtime. Increase the dose by 125 mg every 3 days to 250 mg every 12 hours if needed. The maximum dose is 500 mg daily.

Primidone may decrease blood levels and the effect of several drugs by increasing the activity of liver enzymes that breakdown these drugs. Examples of drugs affected by Primidone include naloxegol (Movantik), roflumilast (Daliresp), lurasidone (Latuda), and many others.

Primidone has not been adequately evaluated in pregnant women. Some other seizure medications have been associated with birth defects. Primidone is excreted in human milk and may cause adverse effects in the infant.

Primidone is available as 50 mg and 250 mg tablets.

Keep this drug stored at room temperature, 20 C - 25 C (68 F - 77 F).

Primidone is available in generic form. You need a prescription from your doctor to obtain this drug.