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 the present study concluded that metformin may be a potential agent for the treatment of epilepsy as well as a protective medicine against cognitive impairment induced by seizures .
### Output Text:
Thus the present study concluded that <span class="chemical">metformin</span> may be a potential agent for the treatment of epilepsy as well as a protective medicine against cognitive impairment induced by seizures . 