ELEVATED PLASMA LEVELS OF THEOPHYLLINE HAVE BEEN REPORTED WITH COMCOMITANT QUINOLONE USE. QUINOLONES INTERFERE WITH THE METABOLISM OF CAFFEIN. QUINOLONES INCLUDING NALIDIXIC, MAY ENHANCE THE EFFECTS OF THE ORAL ANTICOAGULANT WARFARIN OR ITS DERIVATIVES. NITROFURANTOIN INTERFERES WITH THE THERAPEUTIC ACTION OF NALIDIXIC ACID.SERIOUS G.I. TOXICITY IS SEEN WITH MELPHALAN AND NEPHROTOXICITY WITH CYCLOSPORINE.