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: We aimed to compare the risks of hospitalization for AMI and GI bleeding among elderly patients using COX - 2 inhibitors , NS - NSAIDs and acetaminophen .
### Output Text:
We aimed to compare the risks of hospitalization for AMI and GI bleeding among elderly patients using <span class="chemical">COX - 2 inhibitors</span> , NS - NSAIDs and <span class="chemical">acetaminophen</span> . 