In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: While small prospective studies have not revealed any significant drug - drug interaction between warfarin and levofloxacin , several case reports have indicated that levofloxacin may significantly potentiate the anticoagulation effect of warfarin .
### Output Text:
While small prospective studies have not revealed any significant drug - drug interaction between <span class="chemical">warfarin</span> and <span class="chemical">levofloxacin</span> , several case reports have indicated that <span class="chemical">levofloxacin</span> may significantly potentiate the anticoagulation effect of <span class="chemical">warfarin</span> . 