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 rates of events in users of aspirin were similar for all 3 treatment groups and across valdecoxib doses .
### Output Text:
The rates of events in users of <span class="chemical">aspirin</span> were similar for all 3 treatment groups and across <span class="chemical">valdecoxib</span> doses . 