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: Three subjects ( 2 on sertraline and 1 on placebo ) withdrew from the study because of side effects .
### Output Text:
Three subjects ( 2 on <span class="chemical">sertraline</span> and 1 on placebo ) withdrew from the study because of side effects . 