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 risk for those prescribed cephalexin was somewhat lower ( 2 . 0 / 100 , 000 ) .
### Output Text:
The risk for those prescribed <span class="chemical">cephalexin</span> was somewhat lower ( 2 . 0 / 100 , 000 ) . 