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 selective deficit in fear recognition accuracy manifested by the RC group cannot be explained by the subacute effects of cocaine , or ecstasy , because recent and less recent users of these drugs within this group were similarly impaired .
### Output Text:
The selective deficit in fear recognition accuracy manifested by the RC group cannot be explained by the subacute effects of <span class="chemical">cocaine</span> , or <span class="chemical">ecstasy</span> , because recent and less recent users of these drugs within this group were similarly impaired . 