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: Crude and exposure - adjusted incidences of thrombotic events were similar for valdecoxib , NSAIDs , and placebo .
### Output Text:
Crude and exposure - adjusted incidences of thrombotic events were similar for <span class="chemical">valdecoxib</span> , NSAIDs , and placebo . 