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 risk of serious thrombotic events was also similar for each valdecoxib dose .
### Output Text:
The risk of serious thrombotic events was also similar for each <span class="chemical">valdecoxib</span> dose . 