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: Drugs were ceased in six patients because of adverse reactions ; in three of these vancomycin was considered the likely cause .
### Output Text:
Drugs were ceased in six patients because of adverse reactions ; in three of these <span class="chemical">vancomycin</span> was considered the likely cause . 