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 MDMA - free polydrug controls , MDMA polydrug users showed impairments in set shifting and memory updating , and also in social and emotional judgement processes .
### Output Text:
Compared with <span class="chemical">MDMA</span> - free polydrug controls , <span class="chemical">MDMA</span> polydrug users showed impairments in set shifting and memory updating , and also in social and emotional judgement processes . 