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: Strikingly , despite prolonged abstinence ( mean , 4 . 98 ; range , 4 - 9 years ) , past ecstasy users showed few signs of recovery .
### Output Text:
Strikingly , despite prolonged abstinence ( mean , 4 . 98 ; range , 4 - 9 years ) , past <span class="chemical">ecstasy</span> users showed few signs of recovery . 