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: Group 5 received a similar combination of cocaine and diazepam , followed later by 5 mg / kg flumazenil .
### Output Text:
Group 5 received a similar combination of <span class="chemical">cocaine</span> and <span class="chemical">diazepam</span> , followed later by 5 mg / kg <span class="chemical">flumazenil</span> . 