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: With widespread use of antimicrobial agents , however , hepatic injury occurs frequently , and among adverse drug reactions , idiosyncratic reactions are the most serious .
### Output Text:
With widespread use of antimicrobial agents , however , hepatic injury occurs frequently , and among adverse drug reactions , idiosyncratic reactions are the most serious . 