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 , both celecoxib and naproxen seemed to be the least toxic .
### Output Text:
Among users of <span class="chemical">aspirin</span> , both <span class="chemical">celecoxib</span> and <span class="chemical">naproxen</span> seemed to be the least toxic . 