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: Total follow - up on warfarin was 530 years ( mean 28 months ) .
### Output Text:
Total follow - up on <span class="chemical">warfarin</span> was 530 years ( mean 28 months ) . 