The difference between levofloxacin and ofloxacin mesylate is different structure and different side effects. 1. Molecular structure: the molecular structure of levofloxacin is CHClFN₃O₄, while the molecular structure of ofloxacin mesylate is C18H20FN3O4-CH4O3S-H2O. 2. Side effects are different: although the two drugs have the same effect, but the side effects are different, levofloxacin may appear after taking diarrhea, nausea, rash, seizures, joint pain and other side effects, side effects are slightly larger; ofloxacin mesylate relatively speaking, side effects are smaller, and occasional gastrointestinal adverse reactions. As long as the drug will exist certain side effects, so before using the drug need to consult a professional doctor, during the use of drugs also need to strictly quit smoking and alcohol.