If a child vomits after taking medication, does the dose need to be repeated?

My son is 4 years old. He has some episodes of fits. He is on Dilantin syrup, 4 ml daily. I want to know how much time does it take to digest the syrup? If it is vomited within some specified time do I need to repeat the dose?

 

As a general principle, if vomiting occurs within half an hour of taking a drug, or if the vomit reveals part of drug ingested, the drug dose should be repeated.