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: CONCLUSIONS : Given this record of impaired memory and clinically significant levels of depression , impulsiveness , and sleep disturbance , the prognosis for the current generation of ecstasy users is a major cause for concern .
### Output Text:
CONCLUSIONS : Given this record of impaired memory and clinically significant levels of depression , impulsiveness , and sleep disturbance , the prognosis for the current generation of <span class="chemical">ecstasy</span> users is a major cause for concern . 