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: Subsequent treatments included alternative anticoagulants ( 11 patients ) , thrombolytic drugs ( 3 patients ) , inferior vena cava filters ( 3 patients ) and , eventually , warfarin ( 11 patients ) .
### Output Text:
Subsequent treatments included alternative anticoagulants ( 11 patients ) , thrombolytic drugs ( 3 patients ) , inferior vena cava filters ( 3 patients ) and , eventually , <span class="chemical">warfarin</span> ( 11 patients ) . 