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1.
The shitty feeling you get straight after eating a McDonalds meal, usually coupled with a 'still hungry' feeling. Usually due to all the additives and addictive ingredients in the food.

Symptoms include bloatedness, tiredness, feeling sick, feeling ridiculously full but still wanting to pump your face full of food; We call this phantom hunger. Recent studies show that McDonalds Hangovers are most common on Sunday's, with 62% occurring on this day.
Fat Girl: Man I feel aweful after eating that McDonalds but I still want more. I think I may suffer from constant munchies(Phantom Hunger)!

Fat Boy: Nah, thats just the McDonalds hangover kicking in. You are now a Maccy junkie(education)!
by Jade Sulph-Dearing, Mark Hollins November 25, 2007
 
2.
The act of getting really drunk and consuming Mcdonalds late at night only to wake up the next day feeling extra terrible. The event may or may not be recalled but is evident by an intensified hangover like feeling of grossness. A mild-moderate feeling of shame might come upon some individuals as the realization hits them the next morning.
Dude, I had such a great time last night but I wish I would've had a little more self control. Woke up with a nasty Mcdonalds hangover.
by Gandy11 February 26, 2014
 
3.
The feeling you get usually within hours, and sometimes the next day after eating McDonald's. Feelings of extreme stomach pains, sharpness, bloating, headaches, diarrhea, and if you are lucky, anal leakage.

It is not uncommon to go to the bathroom several times after eating McDonald's to go poop. The poop is usually black or greenish.
Ohh man I was in the bathroom 4 times the past hour, damn McDonalds Hangover. I will never eat there again.

Similar to a alochol related hangover you will eat at McDonald's the next week.
by haasmr November 17, 2011