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: Heavy and ex - MDMA + users performed significantly poorer on memory tasks than controls .
### Output Text:
Heavy and ex - <span class="chemical">MDMA</span> + users performed significantly poorer on memory tasks than controls . 