What is the oral dose of Phenytoin?

From the FDA Drug Label

Adult Dosage Divided daily 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. Once-a-day dosage In adults, if seizure control is established with divided doses of three 100-mg Dilantin (extended phenytoin sodium capsules, USP) daily, once-a-day dosage with 300 mg of Dilantin (extended phenytoin sodium capsules, USP) may be considered Loading dose Some authorities have advocated use of an oral loading dose of phenytoin in adults who require rapid steady-state serum levels and where intravenous administration is not desirable Initially, one gram of Dilantin (extended phenytoin sodium capsules, USP) is divided into three doses (400 mg, 300 mg, 300 mg) and administered at two-hour intervals

The oral dose of Phenytoin is:

• Divided daily dosage: 100 mg three times daily, adjusted to suit individual requirements, with a satisfactory maintenance dosage of one capsule three to four times a day (300-400 mg daily)
• Once-a-day dosage: 300 mg daily
• Loading dose: 1 gram divided into three doses (400 mg, 300 mg, 300 mg) administered at two-hour intervals 1

From the Research

Oral Dose of Phenytoin

The oral dose of Phenytoin can vary depending on the patient and the specific condition being treated.

• The mean calculated dose of phenytoin sodium required for a steady state serum concentration of 10-20 μg/ml was 345-400 mg/day 2.
• A single oral loading dose of 15 mg/kg of phenytoin was given to 19 normal medical volunteers, with serum concentrations reaching the therapeutic level (10 micrograms/ml) with an average of 2.62 +/- 1.25 h 3.
• The oral loading doses, calculated from a model incorporating a first order of absorption and Michaelis-Menten elimination kinetics to yield the target phenytoin serum concentration of 15 micrograms/ml were 18.7 mg/kg in males and 24.8 mg/kg in females 3.
• Patients with subtherapeutic phenytoin concentrations who presented within 48 hours of a seizure were randomized to receive either 20 mg/kg of oral phenytoin (PO), divided in maximum doses of 400 mg every two hours 4.
• The dose ranged from 12 to 23 mg/kg body weight during 1 to 12 hours in twenty-five epileptic patients treated with the oral loading dose of phenytoin 5.

Factors Affecting Oral Dose

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

• Sex: The oral loading doses, calculated from a model incorporating a first order of absorption and Michaelis-Menten elimination kinetics to yield the target phenytoin serum concentration of 15 micrograms/ml were 18.7 mg/kg in males and 24.8 mg/kg in females 3.
• Body weight: The dose ranged from 12 to 23 mg/kg body weight during 1 to 12 hours in twenty-five epileptic patients treated with the oral loading dose of phenytoin 5.
• Age: No specific information is available on how age affects the oral dose of Phenytoin in the provided studies.
• Method of administration: Patients that received phenytoin through the naso-gastric tube, in respect to oral administration, required higher doses to obtain therapeutic plasma levels of phenytoin 5.

Adverse Effects

The oral dose of Phenytoin can also be affected by adverse effects, such as:

• Nausea: One patient had mild nausea on the first day of the treatment 5.
• Dizziness: 3 patients had mild dizziness on the first day of the treatment 5.
• Tinitus: 1 patient had tinitus on the first day of the treatment 5.
• Adverse drug events: A total of 17,27, and 32 adverse drug events were observed in the PO, IVP, and IVF groups, respectively, with significantly fewer events in the PO group 4.