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: There was also an excess of MB in warfarin users vs nonusers with ICH ( OR , 2 . 7 ; 95 % CI , 1 . 6 - 4 . 4 ; P < 0 . 001 ) but none in warfarin users with IS / TIA ( OR , 1 . 3 ; 95 % CI , 0 . 9 - 1 . 7 ; P = 0 . 33 ; P difference = 0 . 01 ) .
### Output Text:
There was also an excess of MB in <span class="chemical">warfarin</span> users vs nonusers with ICH ( OR , 2 . 7 ; 95 % CI , 1 . 6 - 4 . 4 ; P < 0 . 001 ) but none in <span class="chemical">warfarin</span> users with IS / TIA ( OR , 1 . 3 ; 95 % CI , 0 . 9 - 1 . 7 ; P = 0 . 33 ; P difference = 0 . 01 ) . 