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: Metformin , the most commonly prescribed antidiabetic oral drug , has antioxidant properties .
### Output Text:
<span class="chemical">Metformin</span> , the most commonly prescribed antidiabetic oral drug , has antioxidant properties . 