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### Input Text: Recently , fenfluramine appetite suppressants became widely used in the United States but were withdrawn in September 1997 because of concerns over adverse effects .
### Output Text:
Recently , <span class="chemical">fenfluramine</span> appetite suppressants became widely used in the United States but were withdrawn in September 1997 because of concerns over adverse effects . 