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: We report 3 cases of serious bleeding complications that appear to be the result of the interaction between warfarin and levofloxacin .
### Output Text:
We report 3 cases of serious bleeding complications that appear to be the result of the interaction between <span class="chemical">warfarin</span> and <span class="chemical">levofloxacin</span> . 