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: Medical records were studied retrospectively to evaluate whether warfarin and warfarin - drug interactions could have caused the cerebral haemorrhage .
### Output Text:
Medical records were studied retrospectively to evaluate whether <span class="chemical">warfarin</span> and <span class="chemical">warfarin</span> - drug interactions could have caused the cerebral haemorrhage . 