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: Recombinant human erythropoietin has been used to manage ribavirin - associated anemia but has other potential disadvantages .
### Output Text:
Recombinant human erythropoietin has been used to manage <span class="chemical">ribavirin</span> - associated anemia but has other potential disadvantages . 