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: A warfarin - drug interaction could have contributed to the haemorrhage in 24 ( 41 % ) of the warfarin patients and in 7 of these ( 12 % ) the bleeding complication was considered being possible to avoid .
### Output Text:
A <span class="chemical">warfarin</span> - drug interaction could have contributed to the haemorrhage in 24 ( 41 % ) of the <span class="chemical">warfarin</span> patients and in 7 of these ( 12 % ) the bleeding complication was considered being possible to avoid . 