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.

Benadryl hangover by brytni102 May 16, 2011

Benedryl Hangover

The effect felt the morning after taking Benedryl or other OTC drug containing diphenhydramine such as Unisom or Tylenol PM in order to induce or aid sleep. Symptoms include inability to wake up, sleeping through alarm clocks, grogginess, stiff neck, loss of memory, and general irritability. There is no known cure.

"I was late to work, I couldn't drive until I worked off this benedryl hangover"

"Dude, after all that Unisom you downed last night, I'm surprised you are still alive!"
"Uuuunnnnggghhhh."

Benedryl Hangover by the banana July 29, 2007