Phenytoin Sodium Dosage
Medically reviewed by Drugs.com. Last updated on Oct 21, 2022.
General Dosing Information
Because of the increased risk of adverse cardiovascular reactions associated with rapid administration, intravenous administration should not exceed 50 mg per minute in adults. In pediatric patients, the drug should be administered at a rate not exceeding 1 to 3 mg/kg/min or 50 mg per minute, whichever is slower.
As non-emergency therapy, phenytoin sodium injection should be administered more slowly as either a loading dose or by intermittent infusion. Because of the risks of cardiac and local toxicity associated with intravenous phenytoin sodium injection, oral phenytoin should be used whenever possible.
Because adverse cardiovascular reactions have occurred during and after infusions, careful cardiac monitoring is needed during and after the administration of intravenous phenytoin sodium injection. Reduction in rate of administration or discontinuation of dosing may be needed.
Because of the risk of local toxicity, intravenous phenytoin sodium injection should be administered directly into a large peripheral or central vein through a large-gauge catheter. Prior to the administration, the patency of the intravenous (IV) catheter should be tested with a flush of sterile saline. Each injection of parenteral phenytoin sodium injection should then be followed by a flush of sterile saline through the same catheter to avoid local venous irritation due to the alkalinity of the solution.
Phenytoin sodium injection can be given diluted with normal saline. The addition of parenteral phenytoin sodium injection to dextrose and dextrose-containing solutions should be avoided due to lack of solubility and resultant precipitation.
Treatment with phenytoin sodium injection can be initiated either with a loading dose or an infusion:
Loading Dose: A loading dose of parenteral phenytoin sodium injection should be injected slowly, not exceeding 50 mg per minute in adults and 1 to 3 mg/kg/min (or 50 mg per minute, whichever is slower) in pediatric patients.
Infusion: For infusion administration, parenteral phenytoin sodium injection should be diluted in normal saline with the final concentration of phenytoin sodium injection in the solution no less than 5 mg/mL. Administration should commence immediately after the mixture has been prepared and must be completed within 1 to 4 hours (the infusion mixture should not be refrigerated). An in-line filter (0.22 to 0.55 microns) should be used.
Parenteral drug products should be inspected visually for particulate matter and discoloration prior to administration, whenever solution or container permit.
The diluted infusion mixture (phenytoin sodium injection plus normal saline) should not be refrigerated. If the undiluted parenteral phenytoin sodium injection is refrigerated or frozen, a precipitate might form: this will dissolve again after the solution is allowed to stand at room temperature. The product is still suitable for use. A faint yellow coloration may develop, however this has no effect on the potency of the solution.
For single-dose only. After opening, any unused product should be discarded.
Monitoring Levels: Trough levels provide information about clinically effective serum level range and are obtained just prior to the patient’s next scheduled dose. Peak levels indicate an individual’s threshold for emergence of dose-related side effects and are obtained at the time of expected peak concentration. Therapeutic effect without clinical signs of toxicity occurs more often with serum total concentrations between 10 and 20 mcg/mL (unbound phenytoin concentrations of 1 to 2 mcg/mL), although some mild cases of tonic-clonic (grand mal) epilepsy may be controlled with lower serum levels of phenytoin. In patients with renal or hepatic disease, or in those with hypoalbuminemia, the monitoring of unbound phenytoin concentrations may be more relevant [ see Dosage and Administration (2.3)].
In adults, a loading dose of 10 to 15 mg/kg should be administered slowly intravenously, at a rate not exceeding 50 mg per minute (this will require approximately 20 minutes in a 70-kg patient).
The loading dose should be followed by maintenance doses of 100 mg orally or intravenously every 6 to 8 hours.
In the pediatric population, a loading dose of 15 to 20 mg/kg of phenytoin sodium injection intravenously will usually produce serum concentrations of phenytoin within the generally accepted serum total concentrations between 10 and 20 mcg/mL (unbound phenytoin concentrations of 1 to 2 mcg/mL). The drug should be injected slowly intravenously at a rate not exceeding 1 to 3 mg/kg/min or 50 mg per minute, whichever is slower.
Continuous monitoring of the electrocardiogram and blood pressure is essential. The patient should be observed for signs of respiratory depression.
Determination of phenytoin serum levels is advised when using phenytoin sodium injection in the management of status epilepticus and in the subsequent establishment of maintenance dosage.
Other measures, including concomitant administration of an intravenous benzodiazepine such as diazepam, or an intravenous short-acting barbiturate, will usually be necessary for rapid control of seizures because of the required slow rate of administration of phenytoin sodium injection.
If administration of parenteral phenytoin sodium injection does not terminate seizures, the use of other anticonvulsants, intravenous barbiturates, general anesthesia, and other appropriate measures should be considered.
Intramuscular administration should not be used in the treatment of status epilepticus because the attainment of peak serum levels may require up to 24 hours.
Non-emergent Loading and Maintenance Dosing
Because of the risks of cardiac and local toxicity associated with intravenous phenytoin sodium injection, oral phenytoin should be used whenever possible. In adults, a loading dose of 10 to 15 mg/kg should be administered slowly. The rate of intravenous administration should not exceed 50 mg per minute in adults and 1 to 3 mg/kg/min (or 50 mg per minute, whichever is slower) in pediatric patients. Slower administration rates are recommended to minimize the cardiovascular adverse reactions. Continuous monitoring of the electrocardiogram, blood pressure, and respiratory function is essential.
The loading dose should be followed by maintenance doses of oral or intravenous phenytoin sodium injection every 6 to 8 hours.
Ordinarily, phenytoin sodium injection should not be given intramuscularly because of the risk of necrosis, abscess formation, and erratic absorption. If intramuscular administration is required, compensating dosage adjustments are necessary to maintain therapeutic serum levels. An intramuscular dose 50% greater than the oral dose is necessary to maintain these levels. When returned to oral administration, the dose should be reduced by 50% of the original oral dose for one week to prevent excessive serum levels due to sustained release from intramuscular tissue sites.
Monitoring serum levels would help prevent a fall into the subtherapeutic range. Serum blood level determinations are especially helpful when possible drug interactions are suspected.
Parenteral Substitution for Oral Phenytoin Therapy
When treatment with oral phenytoin is not possible, IV phenytoin sodium injection can be substituted for oral phenytoin at the same total daily dose. Phenytoin sodium injection capsules are approximately 90% bioavailable by the oral route. Phenytoin is 100% bioavailable by the IV route. For this reason, serum phenytoin concentrations may increase modestly when IV phenytoin is substituted for oral phenytoin sodium therapy. The rate of administration for IV phenytoin sodium injection should be no greater than 50 mg per minute in adults and 1 to 3 mg/kg/min (or 50 mg per minute, whichever is slower) in pediatric patients.
When intramuscular administration may be required, a sufficient dose must be administered intramuscularly to maintain the serum level within the therapeutic range. Where oral dosage is resumed following intramuscular usage, the oral dose should be properly adjusted to compensate for the slow, continuing IM absorption to minimize toxic symptoms [ see Clinical Pharmacology (12.3)].
Serum concentrations should be monitored and care should be taken when switching a patient from the sodium salt to the free acid form. Phenytoin sodium injection extended capsules and parenteral Phenytoin Sodium Injection are formulated with the sodium salt of phenytoin. The free acid form of phenytoin is used in Phenytoin Sodium Injection-125 Suspension and Phenytoin Sodium Injection Infatabs. Because there is approximately an 8% increase in drug content with the free acid form over that of the sodium salt, dosage adjustments and serum level monitoring may be necessary when switching from a product formulated with the free acid to a product formulated with the sodium salt and vice versa.
Dosing in Patients with Renal or Hepatic Impairment or Hypoalbuminemia
Because the fraction of unbound phenytoin is increased in patients with renal or hepatic disease, or in those with hypoalbuminemia, the monitoring of phenytoin serum levels should be based on the unbound fraction in those patients.
Dosing in Geriatrics
Phenytoin clearance is decreased slightly in elderly patients and lower or less frequent dosing may be required [ see Clinical Pharmacology (12.3)].
Dosing during Pregnancy
Decreased serum concentrations of phenytoin may occur during pregnancy because of altered phenytoin pharmacokinetics. Periodic measurement of serum phenytoin concentrations should be performed during pregnancy, and the phenytoin sodium injection dosage should be adjusted as necessary. Postpartum restoration of the original dosage will probably be indicated [ see Use in Specific Populations (8.1)]. Because of potential changes in protein binding during pregnancy, the monitoring of phenytoin serum levels should be based on the unbound fraction.
Dosing in Pediatrics
A loading dose of 15 to 20 mg/kg of phenytoin sodium injection intravenously will usually produce serum concentrations of phenytoin within the generally accepted serum total concentrations between 10 and 20 mcg/mL (unbound phenytoin concentrations of 1 to 2 mcg/mL). The drug should be injected slowly intravenously at a rate not exceeding 1 to 3 mg/kg/min or 50 mg per minute, whichever is slower.
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