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: After normal serum sodium levels returned , further doxorubicin and dexamethasone chemotherapy without vincristine did not produce this complication .
### Output Text:
After normal serum <span class="chemical">sodium</span> levels returned , further <span class="chemical">doxorubicin</span> and <span class="chemical">dexamethasone</span> chemotherapy without <span class="chemical">vincristine</span> did not produce this complication . 