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: Previous studies indicate that ecstasy users have marked and persistent neurocognitive and sleep - related impairments .
### Output Text:
Previous studies indicate that <span class="chemical">ecstasy</span> users have marked and persistent neurocognitive and sleep - related impairments . 