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: Among users of aspirin , they were : 14 , 671 to rofecoxib , 22 , 875 to celecoxib , 9 , 832 to NS - NSAIDs and 38 , 048 to acetaminophen .
### Output Text:
Among users of <span class="chemical">aspirin</span> , they were : 14 , 671 to <span class="chemical">rofecoxib</span> , 22 , 875 to <span class="chemical">celecoxib</span> , 9 , 832 to NS - NSAIDs and 38 , 048 to <span class="chemical">acetaminophen</span> . 