What is the normal dose of Phenytoin?

From the Guidelines

The normal dose of Phenytoin is 20 mg/kg divided in maximum doses of 400 mg every 2 h orally, or 18 mg/kg IV at a maximum rate of 50 mg/min.

Phenytoin Dosing

• The dose of Phenytoin can be administered orally or intravenously, with the oral dose being divided into maximum doses of 400 mg every 2 h 1.
• The intravenous dose should not exceed a rate of 50 mg/min to minimize the risk of adverse effects such as hypotension, bradyarrhythmias, cardiac arrest, and extravasation injuries 1.
• The choice of administration route depends on the discretion of the emergency physician, as there is a lack of evidence to support one route over the other in terms of preventing early recurrent seizure 1.

Important Considerations

• Phenytoin should be diluted in normal saline to avoid precipitation and is incompatible with glucose-containing solutions 1.
• The drug-delivery rate should not exceed 1 mg/kg per min, and heart rate should be monitored, with the rate of infusion reduced if the heart rate decreases by 10 beats per min 1.
• Fosphenytoin is preferred over Phenytoin due to its lower risk of adverse cardiac effects 1.
• It is essential to monitor patients for adverse effects, including hypotension, arrhythmias, and cardiac arrest, especially with rapid infusion 1.

From the FDA Drug Label

The clinically effective serum level is usually 10–20 mcg/mL Adult Dosage Patients who have received no previous treatment may be started on one 100-mg Dilantin (extended phenytoin sodium capsules, USP) three times daily and the dosage then adjusted to suit individual requirements. For most adults, the satisfactory maintenance dosage will be one capsule three to four times a day An increase up to two capsules three times a day may be made, if necessary. Pediatric Dosage Initially, 5 mg/kg/day in two or three equally divided doses, with subsequent dosage individualized to a maximum of 300 mg daily. A recommended daily maintenance dosage is usually 4 to 8 mg/kg.

The normal dose of Phenytoin is:

• Adults: 100 mg three to four times a day, with a maximum of two capsules three times a day, or 300 mg once a day for patients requiring this amount of drug daily.
• Pediatric: Initially, 5 mg/kg/day, with subsequent dosage individualized to a maximum of 300 mg daily, and a recommended daily maintenance dosage of 4 to 8 mg/kg. 2

From the Research

Normal Dose of Phenytoin

The normal dose of Phenytoin can vary depending on the patient's weight, age, and medical condition.

• The dose ranged from 12 to 23 mg/kg body weight during 1 to 12 hours in a study published in 1994 3.
• A study published in 2013 found that the mean administered dose was 14.5 (3.66) mg/kg, with 26 patients (49%) having suboptimal trough free concentrations 4.
• In pediatric patients, the intravenous loading dose of phenytoin has been increased from 18 to 20 mg/kg, with serum phenytoin concentrations measured on 48 occasions from 41 children 5.
• Infants in the first three months of life require doses of 10-20 mg/kg/day to achieve therapeutic serum concentrations, which is higher than the doses required in adults 6.
• In overweight patients, using adjusted body weight (AdjBW) instead of actual body weight (ABW) to calculate the loading dose may be more effective in achieving therapeutic phenytoin levels, with a dose of 20 mg/kg based on AdjBW being more likely to achieve a therapeutic concentration compared to those dosed based on ABW 7.

Factors Affecting Dose

Several factors can affect the dose of Phenytoin required, including:

• Weight: patients with a higher weight may require a lower per kilogram dose to achieve therapeutic concentrations 4.
• Age: infants and children may require higher doses than adults to achieve therapeutic serum concentrations 5, 6.
• Medical condition: patients with certain medical conditions, such as status epilepticus, may require higher doses of Phenytoin to achieve therapeutic concentrations 7.
• Administration method: patients who receive Phenytoin through a naso-gastric tube may require higher doses to obtain therapeutic plasma levels of Phenytoin 3.