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: Various reported side effects of fentanyl administration include chest wall rigidity , hypotension , respiratory depression , and bradycardia .
### Output Text:
Various reported side effects of <span class="chemical">fentanyl</span> administration include chest wall rigidity , hypotension , respiratory depression , and bradycardia . 