“If it weren’t for caffeine, I’d have no personality whatsoever” is a jocular saying about coffee drinkers. The saying has been cited in print since at least 1995 and is of unknown authorship.
     
“If it weren’t for the coffee, I’d have no identifiable personality whatsoever” was said by television talk show host David Letterman in 1994. “‘Ladies and gentlemen, if it weren’t for caffeine, I’d have no personality whatsoever.’—David Letterman”—a misquote of “caffeine” for “coffee”—was posted on the newsgroup comp.infosystems.www.users on June 27, 1995.
 
“As it turns out, 93% of my personality was coffee” is a related saying.
                 
       
Google Groups: comp.infosystems.www.users
FLASH: SlipKnot Wins 2 Ziff-Davis/PC Mag Shareware Awards
Larry Gilbert
Jun 27, 1995, 3:00:00 AM
(...)
Larry Gilbert | “Ladies and gentlemen, if it weren’t for caffeine, I’d
.(JavaScript must be enabled to view this email address) | have no personality whatsoever.”
Seattle, WA, USA |—David Letterman
PGP key available |
 
8 December 1995, Albuquerque (NM) Journal, “Last Minute Stocking Stuffers Never Ordinary” by Nancy Harbert, pg. 24:
You can even choose from apt phrases, such as BURP! or “If it weren’t for caffeine, I’d have no personality whatsoever.” Prices are from $1.75 to $18. Accompanying stamp pads cost from $2 to $14.50.
 
Yahoo! Groups: avenger-sfi
“Carlos & Heidi”
Sun Jul 29, 2001 12:10 am
(...)
If it weren’t for caffeine I’d have no personality whatsoever.
- Unknown

2 November 2002, The Gazette (Colorado Springs, CO), “The Buzz: Talk, tidbits & trivia”:
“If it weren’t for caffeine, I’d have no personality whatsoever.” (Unknown)
     
Google Books
Communication Wins
By Steve Hartkopf
Bloomington, IN: AuthorHouse
2009
Pg. 35:
If it weren’t for caffeine I’d have no personality whatsoever! - Anonymous
     
Washington (DC) Post—ComPost blog
Caffeine killer? Never! The Starbucks defense
By Alexandra Petri | September 21, 2010; 1:21 PM ET
Caffeine makes you psychotic? That’s troubling! In the words of my friend and eminent Canadian Adam Goldenberg, “If it weren’t for caffeine, I’d have no personality whatsoever.”
 
Google Books
Fundamentals of Abnormal Psychology (6th edition)
By Ronald J. Comer
New York, NY: Worth Publishers
2011
Pg. 74:
“If it weren’t for caffeine, I’d have no personality whatsoever.”—Anonymous