Pepsi and Coke are rival soft drink companies. A drug testing riddle is:
 
Q: Why was the Pepsi employee fired?
A: He tested positive for Coke.
 
The joke has been cited in print since at least 2003 and has been printed on several images.
   
 
Google Groups: rec.music.phish
Birthday card to Trey
Matt
9/25/03
So I was in the campus store today to send a thank you card to a friend, when I remembered that Trey’s birthday is coming up (9/30). As long as I was at the store, I picked up a birthday card for him. I picked up this particular one, given Trey’s alleged drug use. On the front is an angry manager yelling at a rank and file employee. From the logos on the wall and on the back of the employee’s jacket,
it is obvious that they both work for Pepsi.
 
The caption:
“You’re fired, Jack.
The lab results came back, and you tested positive for Coke.”
 
The inside of the card says:  “Hope your birthday’s the real thing.”
 
Google Groups: alt.callahans
Re: things to think about . . .
Wesley Struebing
5/19/05
(...)
>>>> If you drink Pepsi at work in the Coke factory, will they fire you?
 
Nah, but if you test positive for Coke at a Pepsi factory, you’ll probably get fired…
 
31 July 2006, Atlanta (GA)

, “The Vent,” pg. D2:
Do you suppose a Pepsi employee could be fired for testing positive for Coke?
     
Google Groups: Blame Tigers and Ahmed’s territorial Theory
Blame Tigers and Ahmed’s territorial Theory
Tigers (and Ahmed) suck
7/29/07
(...)
Why was the Pepsi exec fired???
He tested positive for Coke
 
Twitter
emma mayers‏
@emzie84
I’ve just been fired by Pepsi. I tested positive for Coke.
2:56 AM - 23 Apr 2009
 
Google Books
The Mammoth Book of One-Liners
Edited by Geoff Tibballs
London, UK” Cosntable & Robinson Ltd
2012
Pg. ?:
A Pepsi executive was fired last week. He tested positive for Coke.
 
Google Books
More One Liners, Jokes and Gags
Compiled by Grant Tucker
London, UK: Biteback Publishing Ltd
2013
Pg. ?:
I got fired from my job at Pepsi. I tested positive for Coke.
 
Reddit—Jokes
Why did the Pepsi executive get fired?
submitted July 5, 2016 by noodlesarenotpasta
He tested positive for Coke.
 
Reddit—Dad Jokes
The CEO of Pepsi was fired this morning (self.dadjokes)
submitted February 24, 2017 by camelopardalisx
He tested positive for Coke.
 
Reddit—Jokes
Why was the Pepsi employee fired?
submitted May 6, 2017 by emul4tion
He tested positive for coke.