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: Thrombotic risk was consistently higher for users of aspirin users than nonusers of aspirin ( placebo , 1 . 4 % vs . 0 % ; valdecoxib , 1 . 7 % vs . 0 . 2 % ; NSAIDs , 1 . 9 % vs . 0 . 5 % ) .
### Output Text:
Thrombotic risk was consistently higher for users of <span class="chemical">aspirin</span> users than nonusers of <span class="chemical">aspirin</span> ( placebo , 1 . 4 % vs . 0 % ; <span class="chemical">valdecoxib</span> , 1 . 7 % vs . 0 . 2 % ; NSAIDs , 1 . 9 % vs . 0 . 5 % ) . 