Primidone
Antiseizure medicines
Antiseizure medicines
An old antiseizure medicine that the body turns into phenobarbital. Today it is used mostly for essential tremor and some epilepsies.
Quick facts
- Group
- Antiseizure medicines
- Target
- GABA-A receptor (through its metabolite phenobarbital)
- Used for
- Essential tremor
- Key effects
- Excessive tiredness · Poor coordination · Nausea
- Monitor
- Seizure or tremor control · Mood and suicidal thoughts
- ATC
- N03AA03
No doses: the clinician sets them by the current guideline.
Mechanism of action
The figure shows phenobarbital, the main active metabolite of primidone, bound to the human GABA-A receptor (α1β2γ2) with GABA.
Primidone is a deoxy analogue of phenobarbital. The liver converts it into two active metabolites, phenobarbital and phenylethylmalonamide (PEMA); primidone and both metabolites have antiseizure effects. Much of its effect comes from phenobarbital, which prolongs the opening of GABA-A chloride channels. The exact mechanism of primidone itself is not fully understood.
- Primidone is absorbed and reaches the liver
- It is converted into phenobarbital and PEMA
- Phenobarbital strengthens GABA-A inhibition
- Fewer seizures and less tremor
Indications
- Focal and generalized tonic-clonic seizures (largely replaced by newer medicines) Epilepsy and seizures →
- Essential tremor (with propranolol, a first-line option) Essential tremor →
Adverse effects
Common
- Excessive tiredness, drowsiness and dizziness
- Poor coordination
- Nausea, vomiting and loss of appetite
- Double vision and involuntary eye movements (nystagmus)
- Decreased sexual ability
Contraindications
- Porphyria
- Allergy to primidone or phenobarbital
Key interactions
- Alcohol and other sedatives
- More drowsiness and slower reactions
- Medicines broken down by CYP3A4
- Primidone (like phenobarbital) induces CYP3A4 and can lower their levels
What is monitored
- Phenobarbital blood levels during dose changes
- Seizure or tremor control
- Mood and suicidal thoughts
- Laboratory tests ordered at follow-up
Administration and safety
- Oral tablets, usually several times a day
- Started low and raised slowly
- Never stopped suddenly: it is tapered to avoid withdrawal seizures
- When switching from another antiseizure medicine, one is lowered while the other is raised over weeks
Pregnancy and breastfeeding
Its safety in pregnancy is not established and it shares the risks of phenobarbital; the decision is made with the treating team, and the medicine passes into breast milk.
Nursing care
- Check alertness, gait and fall risk, especially in older adults
- Ask about porphyria and phenobarbital allergy before the first dose
- Teach that drowsiness is common and that alcohol makes it worse
- Watch for mood changes and suicidal thoughts and tell the family what to look for
- If breastfeeding, watch the baby for drowsiness and poor weight gain
Pharmacokinetics
- Absorption
- Completely absorbed by mouth
- Metabolism
- In the liver, to phenobarbital and PEMA (both active)
- Half-life
- Primidone itself is short-lived; phenobarbital stays much longer and builds up slowly
Test yourself
Think of the answer, then tap the question to see it.
What are the two active metabolites of primidone?
Phenobarbital and phenylethylmalonamide (PEMA).
Besides epilepsy, which movement disorder is primidone a first-line treatment for?
Essential tremor (propranolol is the other first-line option).
Which inherited metabolic disease contraindicates primidone?
Porphyria.
Connections
← Back to the class: Antiseizure medicines
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: Primidone: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2026); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026); RCSB PDB - 6X3W: Human GABAA receptor alpha1-beta2-gamma2 subtype in complex with GABA plus phenobarbital (RCSB Protein Data Bank, 2026).
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Representative model, for educational purposes only.