Urine drug screens aren't completely useless, but they have a number of limitations. Here is a table where I have compiled all of the false positive causing drugs I could find (pdf):
Here are my sources:
I started with this paper which was I originally heard on EM Abstracts (Jan 2011):
Brahm NC, Yeager LL, Fox MD, Farmer KC, Palmer TA.
Commonly prescribed medications and potential false-positive urine drug screens.
Am J Health Syst Pharm. 2010 Aug 15;67(16):1344-50.
Special thanks to Jon Cole from Hennepin who made this fantastic video.
Other sources include:
UMHS Guidelines for Clinical Care May 2009
Standridge JB, Adams SM, Zotos AP.
Urine drug screening: a valuable office procedure.
Am Fam Physician. 2010 Mar 1;81(5):635-40.
Reisfield GM, Haddad J, Wilson GR, Johannsen LM, Voorhees KL, Chronister CW, Goldberger BA, Peele JD, Bertholf RL.
Failure of amoxicillin to produce false-positive urine screens for cocaine metabolite.
J Anal Toxicol. 2008 May;32(4):315-8.
Ly BT, Thornton SL, Buono C, Stone JA, Wu AH.
False-positive urine phencyclidine immunoassay screen result caused by interference by tramadol and its metabolites.
Ann Emerg Med. 2012 Jun;59(6):545-7.
Swift RM, Griffiths W, Cammera P.
False positive urine drug screens from quinine in tonic water.
Addict Behav. 1989;14(2):213-5.
Added lamotragine -> PCP
Geraci MJ, Peele J, McCoy SL, Elias B. Phencyclidine false positive induced by lamotrigine (Lamictal®) on a rapid urine toxicology screen. Int J Emerg Med. 2010 Dec; 3(4): 327–331.
Added a few more -> PCP
Phencyclidine (PCP) Test Systems Executive Summary. Chemistry and Toxicology Devices. FDA
2013 Apr 25, Link.