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: Fifteen moderate MDMA users ( < 55 lifetime tablets ) , 22 heavy MDMA + users ( > 55 lifetime tablets ) , 16 ex - MDMA + users ( last tablet > 1 year ago ) and 13 controls were compared on a battery of neuropsychological tests .
### Output Text:
Fifteen moderate <span class="chemical">MDMA</span> users ( < 55 lifetime tablets ) , 22 heavy <span class="chemical">MDMA</span> + users ( > 55 lifetime tablets ) , 16 ex - <span class="chemical">MDMA</span> + users ( last tablet > 1 year ago ) and 13 controls were compared on a battery of neuropsychological tests . 