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: Dexamethasone had no significant effect on the incidence or severity of postoperative nausea and vomiting .
### Output Text:
<span class="chemical">Dexamethasone</span> had no significant effect on the incidence or severity of postoperative nausea and vomiting . 