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: Physicians should be aware of this potential interaction and use caution when prescribing levofloxacin to patients taking warfarin .
### Output Text:
Physicians should be aware of this potential interaction and use caution when prescribing <span class="chemical">levofloxacin</span> to patients taking <span class="chemical">warfarin</span> . 