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: Risks and benefits of COX - 2 inhibitors vs non - selective NSAIDs : does their cardiovascular risk exceed their gastrointestinal benefit ?
### Output Text:
Risks and benefits of <span class="chemical">COX - 2 inhibitors</span> vs non - selective NSAIDs : does their cardiovascular risk exceed their gastrointestinal benefit ? 