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 illicit use of cocaine continues in epidemic proportions and treatment for cocaine overdose remains elusive .
### Output Text:
The illicit use of <span class="chemical">cocaine</span> continues in epidemic proportions and treatment for cocaine overdose remains elusive . 