Research Maximum Lethal Dose (MLD) of Benadryl. I attempted using a massive overdose of Benadryl back in January and woke up at some point during the night and vomited/peed my pants. I guess this could be considered a trial run. Anyhow, if you've got antiemetics, take a good, generous dose of what you've got (Zofran, Metoclopramide, etc.) and maybe use a blender to crush the benadryl into some sort of decent tasting shake or smoothie. My mistake was crushing the pills (I believe I crushed up about 200 or so, maybe more) and only mixed the powder with water. If didn't taste good at all, but it did knock me out. It would've worked had I chocked on my vomit (I didn't take antiemetics beforehand, I don't know why as I have plenty). Not that choking on one's own vomit is ideal by any means, but yeah...it was a close call. I didn't go to a hospital afterwards or even tell my psychiatrist at the time, but did confide in my partner.
If I were to attempt this again, I'd definitely take antiemetics and again, use a blender and make it more palatable.
All this said, OTC fatal drug overdoses are rare, but not impossible.