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: The aim of the study was to investigate the effects of moderate and heavy MDMA use on cognitive function , as well as the effects of long - term abstention from MDMA , in subjects genotyped for 5 - HTTLPR .
### Output Text:
The aim of the study was to investigate the effects of moderate and heavy <span class="chemical">MDMA</span> use on cognitive function , as well as the effects of long - term abstention from <span class="chemical">MDMA</span> , in subjects genotyped for 5 - HTTLPR . 