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: OBJECTIVES : The present study aimed to be the largest to date in sample size and 5HT - related behaviors ; the first to compare present ecstasy users with past users after an abstinence of 4 or more years , and the first to include robust controls for other recreational substances .
### Output Text:
OBJECTIVES : The present study aimed to be the largest to date in sample size and 5HT - related behaviors ; the first to compare present <span class="chemical">ecstasy</span> users with past users after an abstinence of 4 or more years , and the first to include robust controls for other recreational substances . 