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 use of aspirin for primary prevention increases the risk for major bleeding events , primarily gastrointestinal bleeding events , in both men and women .
### Output Text:
The use of <span class="chemical">aspirin</span> for primary prevention increases the risk for major bleeding events , primarily gastrointestinal bleeding events , in both men and women . 