Can barbiturates effect an eeg in a teen if the giver does not know the med is in your system ?

Possibly: The eeg will show "slowing" or other effects that may be due to phenobarbital. Is this a historically pertinent question- like "i just was thinking?" or is this a contemporaneous problem? If you've not had a seizure in 14 years, you're probably not going to get another. The phenobarb may have influenced the interpretation of the eeg back then.