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: Given the onset of his jaundice 2 wk after the initiation of metformin , we believe that this case represents an example of metformin - associated hepatotoxicity , the first such case reported .
### Output Text:
Given the onset of his jaundice 2 wk after the initiation of <span class="chemical">metformin</span> , we believe that this case represents an example of <span class="chemical">metformin</span> - associated hepatotoxicity , the first such case reported . 