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: PATIENTS : 46 patients who used fenfluramine or dexfenfluramine for 14 days or more and had echocardiograms obtained before therapy .
### Output Text:
PATIENTS : 46 patients who used <span class="chemical">fenfluramine</span> or <span class="chemical">dexfenfluramine</span> for 14 days or more and had echocardiograms obtained before therapy . 