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: In contrast , no evidence of memory impairment was observed in moderate MDMA users .
### Output Text:
In contrast , no evidence of memory impairment was observed in moderate <span class="chemical">MDMA</span> users . 