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: Clinicians should be aware that tenofovir may raise the risk of renal failure during prolonged administration of vancomycin .
### Output Text:
Clinicians should be aware that <span class="chemical">tenofovir</span> may raise the risk of renal failure during prolonged administration of <span class="chemical">vancomycin</span> . 