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: BACKGROUND : We compared the efficacy and safety of olanzapine vs placebo for the treatment of acute bipolar mania .
### Output Text:
BACKGROUND : We compared the efficacy and safety of <span class="chemical">olanzapine</span> vs placebo for the treatment of acute bipolar mania . 