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: Risk for valvular heart disease among users of fenfluramine and dexfenfluramine who underwent echocardiography before use of medication .
### Output Text:
Risk for valvular heart disease among users of <span class="chemical">fenfluramine</span> and <span class="chemical">dexfenfluramine</span> who underwent echocardiography before use of medication . 