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: OBJECTIVE : To determine the risk for new or worsening valvular abnormalities among users of fenfluramine or dexfenfluramine who underwent echocardiography before they began to take these medications .
### Output Text:
OBJECTIVE : To determine the risk for new or worsening valvular abnormalities among users of <span class="chemical">fenfluramine</span> or <span class="chemical">dexfenfluramine</span> who underwent echocardiography before they began to take these medications . 