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: Compared with present ecstasy users , the past users showed no change for ten measures , increased impairment for two measures , and improvement on just one measure .
### Output Text:
Compared with present <span class="chemical">ecstasy</span> users , the past users showed no change for ten measures , increased impairment for two measures , and improvement on just one measure . 