From erowid:

"Higher doses are often accompanied by hallucinations (seeing and interacting with people, creatures, or objects that aren't there, or NOT seeing things that are actually there, like cars) and dissociation."

Also read these experiences:

Personally, it sounds like a nasty drug which mainly gives unpleasant "trips." I have never tried it in high doses myself, nor do I plan to.
Logs Reviewed by Logs on . wtf does benadryll do? i have no idea what benadryll does to get u tripped up? i can't exactly google or even jeeves, "What does benadryll do in high doses" so i'll ask u guys. Is it basicly like triple c's or no? Rating: 5