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: BACKGROUND : Because uncontrolled echocardiographic surveys suggested that up to 30 % to 38 % of users of fenfluramine and dexfenfluramine had valvular disease , these drugs were withdrawn from the market .
### Output Text:
BACKGROUND : Because uncontrolled echocardiographic surveys suggested that up to 30 % to 38 % of users of <span class="chemical">fenfluramine</span> and <span class="chemical">dexfenfluramine</span> had valvular disease , these drugs were withdrawn from the market . 