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 : Concerns about possible risks of switching to mania associated with antidepressants continue to interfere with the establishment of an optimal treatment paradigm for bipolar depression .
### Output Text:
BACKGROUND : Concerns about possible risks of switching to mania associated with <span class="chemical">antidepressants</span> continue to interfere with the establishment of an optimal treatment paradigm for bipolar depression . 