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: Effects of the cyclooxygenase - 2 specific inhibitor valdecoxib versus nonsteroidal antiinflammatory agents and placebo on cardiovascular thrombotic events in patients with arthritis .
### Output Text:
Effects of the cyclooxygenase - 2 specific inhibitor <span class="chemical">valdecoxib</span> versus nonsteroidal antiinflammatory agents and placebo on cardiovascular thrombotic events in patients with arthritis . 