Benadryl hangover
The period of time when Benadryl is no longer effective against the symptoms it's supposed to hide, but you're still groggy, unfocused, and unable to think clearly.
I took a Benadryl last night to keep my allergies from not allowing me to sleep, but this morning my eyes itch, I'm sneezing, and I still can't think straight. This is the worst Benadryl hangover ever.