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 : The widespread use of benzodiazepines has led to increasing recognition of their unwanted effects .
### Output Text:
BACKGROUND : The widespread use of <span class="chemical">benzodiazepines</span> has led to increasing recognition of their unwanted effects . 