Phenytoin Pharmacology

Phenytoin is one of the most widely prescribed anticonvulsant drugs – and is routinely tested on the NAPLEX exam. Here in this review of phenytoin pharmacology, we learn more about its indications, mechanism of action, side effects, and detailed dosage information.

Phenytoin was established in 1938, then known as diphenylhydantoin.

Phenytoin is a hydantoin derivative drug that works by decreasing abnormal electrical activity in your brain – specifically by acting as a non-specific sodium channel blocker. Most commonly, it is used to treat generalized tonic-clonic seizures and partial seizures with complex symptoms. It can also be used for:

• Focal seizures
• Status epilepticus
• Prevention or treatment of seizures due to surgery
• Prevention or treatment of seizures due to severe head injury

Phenytoin is an effective drug with low toxicity. For some patients, there may be a genetic inefficiency to metabolize phenytoin, leading to increased serum concentrations and a heightened risk of toxicity. As phenytoin is a narrow therapeutic index drug, it is imperative that serum concentrations are measured throughout therapy.

Toxicity signs of phenytoin include:

• Slurred speech
• Ataxia
• Nausea and vomiting
• Lethargy

Side effects

Phenytoin is associated with a substantial range of potential side effects, among them include:

• Ataxia
• CNS depression
• Headache
• Slurred speech
• Dizziness
• Drowsiness
• Nausea and vomiting
• Gingival hyperplasia
• Rash
• Hypotension
• Nystagmus
• Mental confusion

Phenytoin can be administered orally or parenterally. Due to the risk of GI effects, orally administered phenytoin is recommended to be taken with food.

Dosage information

Anticonvulsants drugs are used to control seizures. They do not cure epilepsy. The dose of phenytoin is based on various parameters, including:

• Patient’s response to therapy
• Occurrence and frequency of adverse drug reactions

Dosage may need to be adjusted, particularly during the initial period. If phenytoin is unable to provide complete relief, an additional anticonvulsant drug may be used.

For tonic-clonic or focal seizures

For children aged between 1 month to 11 years.

• By mouth, 1.5–2.5 mg/kg twice daily to 2.5–5 mg/kg twice daily. The dose should be adjusted according to response. The maximum dose of phenytoin per dose is 7.5mg/kg twice daily.
• A daily dose of phenytoin should not exceed 300 mg.

For children aged between 12 to 17 years;

• By mouth, 75–150 mg twice daily to 150–200 mg twice daily. The dose should be adjusted according to response.
• Daily dose of phenytoin should not exceed 300 mg.

Prevention and treatment of seizures during or following neurosurgery or severe head injury.

For children

• By mouth, 2.5 mg/kg twice daily to 4–8 mg/kg daily.
• Maximum dose up to 300mg/day.

For adults

• By mouth, 3–4 mg/kg daily or 150 to 300 mg once daily or in two divided doses.
• The standard maintenance dose is 200–500 mg daily.

Status epilepticus.

For children aged 1 month to 11 years.

Intravenously, a loading dose of 20 mg/kg, followed by a maintenance dose 2.5–5 mg/kg twice daily.

For children aged 12–17 years.

Intravenously, a loading dose of 20 mg/kg, followed by a maintenance dose up to 100 mg 3–4 times a day.

For adults.

Intravenously, a loading dose of 20 mg/kg, followed by a maintenance dose up to 100 mg every 6–8 hours.

That concludes our review of phenytoin pharmacology! Check back to our blog at NAPLEX Study Guide soon for more exclusive content to help you master the NAPLEX exam and become a qualified pharmacist!