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: The antidiabetic drug metformin can diminish apoptosis induced by oxidative stress in endothelial cells and prevent vascular dysfunction even in nondiabetic patients .
### Output Text:
The antidiabetic drug <span class="chemical">metformin</span> can diminish apoptosis induced by oxidative stress in endothelial cells and prevent vascular dysfunction even in nondiabetic patients . 