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: Cerebral haemorrhage induced by warfarin - the influence of drug - drug interactions .
### Output Text:
Cerebral haemorrhage induced by <span class="chemical">warfarin</span> - the influence of drug - drug interactions . 