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: The AMI / GI toxicity of celecoxib was similar to that of acetaminophen and seemed to be better than those of rofecoxib and NS - NSAIDs .
### Output Text:
The AMI / GI toxicity of <span class="chemical">celecoxib</span> was similar to that of <span class="chemical">acetaminophen</span> and seemed to be better than those of <span class="chemical">rofecoxib</span> and NS - NSAIDs . 