We often see seizure patients on phenytoin therapy who have subtherapeutic levels. Most patients do not require intravenous loading and can be adequately managed with oral treatment.
To estimate what dose to prescribe, use the following equation: [0.7 x IBW x (15 - current level)]. For example if a 70 kg patient has a level of 8 mcg/mL (mg/L), we would need ~400 mg loading dose to achieve a level of 15.
Phenytoin is known for its erratic absorption and propensity for causing GI upset with doses too high. The recommended strategy is to avoid administering more than 400 mg at one time and separate the doses by 2 hours. This would take three doses over 4 hours for a 1 gm load.
In the ED, an effective strategy for a 1 gm oral load is 500 mg now and 500 mg in 2 hours at discharge. Patients tolerate it well, it cuts down on ED length of stay, and still achieves therapeutic levels. Remember that an oral suspension formulation is also available.