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 also protected the kidney from histological damage 6 days after gentamicin administration .
### Output Text:
<span class="chemical">Metformin</span> also protected the kidney from histological damage 6 days after <span class="chemical">gentamicin</span> administration . 