Some antibiotics may cause false positives for opiates:
"A total of 13 quinolone antibiotics were solubilized to
represent expected urinary concentrations for testing via 5
different commercial immonoassays (levofloxacin [levaquin],
ofloxacin [floxin], gatifloxacin, enoxacin, moxifloxacin,
sparfloxacin, pefloxacin, trovafloxacin, lomefloxacin,
ciprofloxacin [cipro], cinafloxacin, norfloxacin, and nalidixic
acid. The assays included EMIT II reagents, AxSYM flourescence
polarization immunoassay, CEDIA, Roche Abuscreen OnLine reagents,
and Beckman opiate reagents. In vitro testing against a positive
concentration of morphine (300 ng/mL) revealed that 9 of the
quinolones (levofloxacin, ofloxacin, perfloxacin, enoxacin,
gatifloxacin, lomefloxacin, moxifloxacin, ciprofloxacin, and
norfloxacin) caused false positives in at least one of the assay
systems. The quinolones that cross reacted most often included
levofloxacin (4/5), ofloxacin (4/5), perfloxacin (3/5) and enoxacin
(2/5)."
Baden LR Horowitz B, Jacoby H Eliopoulos GM Quinolones and false
positive urine screeing for opiates by immunoassay technology JAMA
2001; 286:3115-3119.