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: While the mechanisms underlying cocaine ' s rewarding effects have been studied extensively , less attention has been paid to the unpleasant behavioral states induced by cocaine , such as anxiety .
### Output Text:
While the mechanisms underlying <span class="chemical">cocaine</span> ' s rewarding effects have been studied extensively , less attention has been paid to the unpleasant behavioral states induced by <span class="chemical">cocaine</span> , such as anxiety . 