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: Possible contributing factors may have been concomitant antidepressant use and unaccustomed physical activity .
### Output Text:
Possible contributing factors may have been concomitant <span class="chemical">antidepressant</span> use and unaccustomed physical activity . 