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: Combining the two fentanyl groups revealed further significant benefits from the avoidance of opioids , reducing postoperative nausea and vomiting and nausea prior to discharge from 35 % and 33 % to 22 % and 19 % ( P = 0 . 049 and P = 0 . 035 ) , respectively , while nausea in the first 24 h was decreased from 42 % to 27 % ( P = 0 . 034 ) .
### Output Text:
Combining the two <span class="chemical">fentanyl</span> groups revealed further significant benefits from the avoidance of opioids , reducing postoperative nausea and vomiting and nausea prior to discharge from 35 % and 33 % to 22 % and 19 % ( P = 0 . 049 and P = 0 . 035 ) , respectively , while nausea in the first 24 h was decreased from 42 % to 27 % ( P = 0 . 034 ) . 