Phenytoin : Timing of Maintenance Dose

Loading Dose

IV Loading

• Loading dose: 20 mg/kg at a maximum rate of 50 mg/minute; if necessary, may give an additional dose of 5 to 10 mg/kg 10 minutes after the loading dose (NCS [Brophy 2012]) [Neurocritical Care Society recommendation for status epilepticus -adult/children]

Oral loading:

• Give in 3 to 4 divided doses at q2h intervals.
• Divided doses increase bioavailability as well as decrease potential for GI side effects such as N&V
•  The maximum single oral dose should not exceed 400 mg in order to minimize GI side effects and also increase absorption (decrease likelihood of concretions)

Timing of Maintenance Dose

• Half-life: 7 to 42 hours (average = 24 hours)
• The maintenance dose is started 18-24 hours after the loading dose
• Phenytoin can be administered at 12 hours when the 2-hour level is satisfactory or earlier when the 2-hour level indicates that a subtherapeutic level will occur
•  2 to 4 mg/kg every 12 hrs, commencing 12 hours after the loading dose ( South Australian Neonatal Medication Guidelines)
• Maintenance intravenous phenytoin therapy of 3-5mg/kg/day three times per day should be commenced 12 – 24 hours after loading dose (NHS Tayside).

References:

1. http://www.globalrph.com/pheninfo.htm
2. www.uptodate.com
3. https://www.ncbi.nlm.nih.gov/pubmed/1930417
4. http://adc.bmj.com/content/100/6/e1.11
5.  phenytoin. South Australian Maternal & Neonatal Clinical Network.
6. http://www.icid.salisbury.nhs.uk/MedicinesManagement/Guidance/Generalguidance/Pages/Phenytoin-LoadingDose(Adults).aspx
7. NHS Tayside . Phenytoin Prescribing and Monitoring Guideline