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: Thus , our study suggests that pleiotropic effects of metformin can lessen gentamicin nephrotoxicity and improve mitochondrial homeostasis .
### Output Text:
Thus , our study suggests that pleiotropic effects of <span class="chemical">metformin</span> can lessen <span class="chemical">gentamicin</span> nephrotoxicity and improve mitochondrial homeostasis . 