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: RESULTS : Omission of fentanyl did not reduce the overall incidence of postoperative nausea and vomiting , but did reduce the incidence of vomiting and / or moderate to severe nausea prior to discharge from 20 % and 17 % with fentanyl and fentanyl - dexamethasone , respectively , to 5 % ( P = 0 . 013 ) .
### Output Text:
RESULTS : Omission of <span class="chemical">fentanyl</span> did not reduce the overall incidence of postoperative nausea and vomiting , but did reduce the incidence of vomiting and / or moderate to severe nausea prior to discharge from 20 % and 17 % with <span class="chemical">fentanyl</span> and <span class="chemical">fentanyl</span> - <span class="chemical">dexamethasone</span> , respectively , to 5 % ( P = 0 . 013 ) . 