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: These data lend further support to the proposal that cognitive processes mediated by the prefrontal cortex may be impaired by recreational ecstasy use .
### Output Text:
These data lend further support to the proposal that cognitive processes mediated by the prefrontal cortex may be impaired by recreational <span class="chemical">ecstasy</span> use . 