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: We suggest that sleep deprivation may add to the risk of bupropion - associated seizures .
### Output Text:
We suggest that sleep deprivation may add to the risk of <span class="chemical">bupropion</span> - associated seizures . 