Primidone

Primidone is used alone or with other medications to control seizures. Controlling and reducing seizures lets you do more of your normal daily activities, reduces your risk of harm when you lose consciousness, and lessens your risk for a possibly life-threatening condition of frequent, repeated seizures. Primidone belongs to a class of drugs known as barbiturate anticonvulsants. It works by controlling the abnormal electrical activity in the brain that occurs during a seizure.

• Primidone is available under the following different brand names: Mysoline.

Adult and Pediatric Dosage Forms and Strengths

Tablets

• 50 mg
• 250 mg

Dosage Considerations – Should be Given as Follows:

Seizures (Psychomotor, Focal, and Grand Mal)

Adult:

• Initial: 100-125 mg orally at bedtime for 3 days, THEN
• 100-125 mg twice daily for 3 days, THEN
• 100-125 mg three times daily for 3 days, THEN
• 250 mg three to four times daily; not to exceed 2 g/day

Pediatric::

• Initial: 50 mg orally at bedtime for 3 days, THEN
• 50 mg twice daily for 3 days, THEN
• 100 mg twice daily for 3 days, THEN
• After 9th day: 125-250 mg orally three times daily, OR 10-25 mg/kg/day divided every 8 hours orally

Dosing considerations

• May take with food
• Monitor: Complete blood count (CBC), SMA-12 every 6 months
• Do not exceed 2 g/day
• May control grand mal seizures refractory to other anticonvulsants
• Do not abruptly discontinue, due to the risk of precipitation of status epilepticus
• Therapeutic efficacy of dosage regimen may take several weeks to assess

Essential Tremor

• 12.5-25 mg/day orally at bedtime
• Increase dose over 2-3 weeks to 250 mg orally once/day or divided twice daily; do not exceed 750 mg/day

Partial Seizures

• 125 mg orally at bedtime
• Increase dose every 3 days by 125 mg if needed to 250 mg every 12 hours; do not exceed 500 mg/day

Common side effects of primidone include:

• Slow or slurred speech
• Numbness and tingling
• Loss of control of bodily movements
• Spinning sensation (vertigo)
• Blurred vision
• Skin rash
• Loss of appetite
• Impotence
• Loss of interest in sex

Pediatric patients:

• Paradoxical excitement
• Hyperactivity

Geriatric patients:

• Excitement
• Confusion
• Depression

Adverse effects of primidone that decrease with continued treatment include:

• Drowsiness
• Loss of control of bodily movements
• Irritability
• Headache
• Restlessness
• Irregular, repetitive eye movements
• Dizziness
• Spinning sensation (vertigo)

Other side effects of primidone include:

• Acute psychosis (rare)
• Nausea
• Vomiting
• Constipation
• Diarrhea
• Megaloblastic (folate-deficiency) anemia
• Lover damage
• Low blood calcium (hypocalcemia)
• Rickets (rare)
• Osteomalacia (rare)
• Rash
• Stevens-Johnson syndrome (rare)
• Easy bruising or bleeding
• Unusual weakness

This document does not contain all possible side effects and others may occur. Check with your physician for additional information about side effects.

If your doctor has directed you to use this medication, your doctor or pharmacist may already be aware of any possible drug interactions and may be monitoring you for them. Do not start, stop, or change the dosage of any medicine before checking with your doctor, health care provider, or pharmacist first.

• Severe interactions of primidone include:
  • artemether/lumefantrine
  • cariprazine
  • cobimetinib
  • daclatasvir
  • dienogest/estradiol valerate
  • elbasvir/grazoprevir
  • elvitegravir/cobicistat/emtricitabine/tenofovir DF
  • lumacaftor/ivacaftor
  • lumefantrine
  • lurasidone
  • naloxegol
  • ombitasvir/paritaprevir/ritonavir
  • ombitasvir/paritaprevir/ritonavir and dasabuvir
  • panobinostat
  • pirfenidone
  • praziquantel
  • regorafenib
  • roflumilast
  • sodium oxybate
  • vandetanib
• Primidone has serious interactions with at least 85 different drugs.
• Primidone has moderate interactions with at least 357 different drugs.
• Primidone has mild interactions with at least 154 different drugs.

This information does not contain all possible interactions or adverse effects. Therefore, before using this product, tell your doctor or pharmacist of all the products you use. Keep a list of all your medications with you, and share this information with your doctor and pharmacist. Check with your health care professional or doctor for additional medical advice, or if you have health questions, concerns or for more information about this medicine.

This medication contains primidone. Do not take Mysoline if you are allergic to primidone or any ingredients contained in this drug.

Keep out of reach of children. In case of overdose, get medical help or contact a Poison Control Center immediately.

Contraindications

• Porphyria
• Hypersensitivity to phenobarbital

Effects of Drug Abuse

• Side effects of primidone overdose (OD) include cardiac damage, death, depression, low blood pressure (hypotension), nausea/vomiting, nervousness, paralysis, respiratory failure, seizures, shortness of breath, slow heart rate, or spasms.

Short-Term Effects

• See "What Are Side Effects Associated with Using Primidone?"

Long-Term Effects

• See "What Are Side Effects Associated with Using Primidone?"

Cautions

• Do not interchange brands, due to bioequivalence issues.
• May cause hyperexcitability, especially in children.
• When replacing another anti-seizure drug, gradually increase the dosage of primidone while gradually decreasing the dosage of other drugs over at least 2 weeks.
• Withdraw gradually, due to the risk of precipitation of status epilepticus.
• A limited number of reports indicate maintaining a serum concentration of 5-12 mcg/mL is necessary.
• Inhibits transplacental vitamin K transport, leading to increased risk of fatal hemorrhage.
• May render oral contraceptive pills ineffective.

Pregnancy and Lactation

• Use primidone during pregnancy only in LIFE-THREATENING emergencies when no safer drug is available. There is positive evidence of human fetal risk. Continue use if pregnant; consider vitamin K supplementation for 1 month before birth. Consult your doctor.
• Primidone is distributed in breast milk. Discontinue if drug effects occur in a nursing infant.