Keywords: phenytoin, anticonvulsants, loading dose (PubMed Search)
Phenytoin po Phenytoin IV Fosphenytoin
Time to therapeutic 6.4 hrs 1.7 hrs 1.3 hrs
Adverse Events 0.69/pt 1.86/pt 1.87/pt
Also to take into account is that the adverse events with IV phenytoin include soft-tissue necrosis if there is extravasation of infusion. The cardiotoxicity seen with phenytoin and fosphenytoin is largely due to the propylene glycol diluent and thus not seen with oral loading or even in oral overdosing.
You decide, at least you have the data to properly evaluate the risk:benefit ratio.