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: The most common adverse effects were nausea and vomiting in the flumazenil group and nausea and injection - site pain in the placebo group .
### Output Text:
The most common adverse effects were nausea and vomiting in the <span class="chemical">flumazenil</span> group and nausea and injection - site pain in the placebo group . 