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: The annual incidence of warfarin - related bleeding at Brigham and Women ' s Hospital increased from 0 . 97 / 1 , 000 patient admissions in the first time period ( January 1995 to October 1998 ) to 1 . 19 / 1 , 000 patient admissions in the second time period ( November 1998 to August 2002 ) of this study .
### Output Text:
The annual incidence of <span class="chemical">warfarin</span> - related bleeding at Brigham and Women ' s Hospital increased from 0 . 97 / 1 , 000 patient admissions in the first time period ( January 1995 to October 1998 ) to 1 . 19 / 1 , 000 patient admissions in the second time period ( November 1998 to August 2002 ) of this study . 