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: CONCLUSION : The frequency of mood switching associated with acute antidepressant therapy may be reduced by lithium treatment .
### Output Text:
CONCLUSION : The frequency of mood switching associated with acute <span class="chemical">antidepressant</span> therapy may be reduced by <span class="chemical">lithium</span> treatment . 